diff --git a/src/merkleblock.h b/src/merkleblock.h
--- a/src/merkleblock.h
+++ b/src/merkleblock.h
@@ -52,7 +52,7 @@
 class CPartialMerkleTree {
 protected:
     /** the total number of transactions in the block */
-    unsigned int nTransactions;
+    uint32_t nTransactions;
 
     /** node-is-parent-of-matched-txid bits */
     std::vector<bool> vBits;
@@ -67,7 +67,7 @@
      * Helper function to efficiently calculate the number of nodes at given
      * height in the merkle tree.
      */
-    unsigned int CalcTreeWidth(int height) const {
+    size_t CalcTreeWidth(int height) const {
         return (nTransactions + (1 << height) - 1) >> height;
     }
 
@@ -75,14 +75,13 @@
      * Calculate the hash of a node in the merkle tree (at leaf level: the
      * txid's themselves)
      */
-    uint256 CalcHash(int height, unsigned int pos,
-                     const std::vector<uint256> &vTxid);
+    uint256 CalcHash(int height, size_t pos, const std::vector<uint256> &vTxid);
 
     /**
      * Recursive function that traverses tree nodes, storing the data as bits
      * and hashes.
      */
-    void TraverseAndBuild(int height, unsigned int pos,
+    void TraverseAndBuild(int height, size_t pos,
                           const std::vector<uint256> &vTxid,
                           const std::vector<bool> &vMatch);
 
@@ -91,10 +90,9 @@
      * hashes produced by TraverseAndBuild. It returns the hash of the
      * respective node and its respective index.
      */
-    uint256 TraverseAndExtract(int height, unsigned int pos,
-                               unsigned int &nBitsUsed, unsigned int &nHashUsed,
-                               std::vector<uint256> &vMatch,
-                               std::vector<unsigned int> &vnIndex);
+    uint256 TraverseAndExtract(int height, size_t pos, size_t &nBitsUsed,
+                               size_t &nHashUsed, std::vector<uint256> &vMatch,
+                               std::vector<size_t> &vnIndex);
 
 public:
     /** serialization implementation */
@@ -109,13 +107,13 @@
             READWRITE(vBytes);
             CPartialMerkleTree &us = *(const_cast<CPartialMerkleTree *>(this));
             us.vBits.resize(vBytes.size() * 8);
-            for (unsigned int p = 0; p < us.vBits.size(); p++) {
+            for (size_t p = 0; p < us.vBits.size(); p++) {
                 us.vBits[p] = (vBytes[p / 8] & (1 << (p % 8))) != 0;
             }
             us.fBad = false;
         } else {
             vBytes.resize((vBits.size() + 7) / 8);
-            for (unsigned int p = 0; p < vBits.size(); p++) {
+            for (size_t p = 0; p < vBits.size(); p++) {
                 vBytes[p / 8] |= vBits[p] << (p % 8);
             }
             READWRITE(vBytes);
@@ -137,7 +135,7 @@
      * root, or 0 in case of failure.
      */
     uint256 ExtractMatches(std::vector<uint256> &vMatch,
-                           std::vector<unsigned int> &vnIndex);
+                           std::vector<size_t> &vnIndex);
 };
 
 /**
@@ -159,7 +157,7 @@
     CPartialMerkleTree txn;
 
     /** Public only for unit testing and relay testing (not relayed) */
-    std::vector<std::pair<unsigned int, uint256>> vMatchedTxn;
+    std::vector<std::pair<size_t, uint256>> vMatchedTxn;
 
     /**
      * Create a Merkle proof according to a bloom filter. Note
diff --git a/src/merkleblock.cpp b/src/merkleblock.cpp
--- a/src/merkleblock.cpp
+++ b/src/merkleblock.cpp
@@ -52,7 +52,7 @@
     txn = CPartialMerkleTree(vHashes, vMatch);
 }
 
-uint256 CPartialMerkleTree::CalcHash(int height, unsigned int pos,
+uint256 CPartialMerkleTree::CalcHash(int height, size_t pos,
                                      const std::vector<uint256> &vTxid) {
     // we can never have zero txs in a merkle block, we always need the
     // coinbase tx if we do not have this assert, we can hit a memory
@@ -77,13 +77,13 @@
     return Hash(BEGIN(left), END(left), BEGIN(right), END(right));
 }
 
-void CPartialMerkleTree::TraverseAndBuild(int height, unsigned int pos,
+void CPartialMerkleTree::TraverseAndBuild(int height, size_t pos,
                                           const std::vector<uint256> &vTxid,
                                           const std::vector<bool> &vMatch) {
     // Determine whether this node is the parent of at least one matched txid.
     bool fParentOfMatch = false;
-    for (unsigned int p = pos << height;
-         p < (pos + 1) << height && p < nTransactions; p++) {
+    for (size_t p = pos << height; p < (pos + 1) << height && p < nTransactions;
+         p++) {
         fParentOfMatch |= vMatch[p];
     }
 
@@ -101,10 +101,11 @@
     }
 }
 
-uint256 CPartialMerkleTree::TraverseAndExtract(
-    int height, unsigned int pos, unsigned int &nBitsUsed,
-    unsigned int &nHashUsed, std::vector<uint256> &vMatch,
-    std::vector<unsigned int> &vnIndex) {
+uint256 CPartialMerkleTree::TraverseAndExtract(int height, size_t pos,
+                                               size_t &nBitsUsed,
+                                               size_t &nHashUsed,
+                                               std::vector<uint256> &vMatch,
+                                               std::vector<size_t> &vnIndex) {
     if (nBitsUsed >= vBits.size()) {
         // Overflowed the bits array - failure
         fBad = true;
@@ -169,7 +170,7 @@
 CPartialMerkleTree::CPartialMerkleTree() : nTransactions(0), fBad(true) {}
 
 uint256 CPartialMerkleTree::ExtractMatches(std::vector<uint256> &vMatch,
-                                           std::vector<unsigned int> &vnIndex) {
+                                           std::vector<size_t> &vnIndex) {
     vMatch.clear();
 
     // An empty set will not work
@@ -198,7 +199,7 @@
     }
 
     // traverse the partial tree.
-    unsigned int nBitsUsed = 0, nHashUsed = 0;
+    size_t nBitsUsed = 0, nHashUsed = 0;
     uint256 hashMerkleRoot =
         TraverseAndExtract(nHeight, 0, nBitsUsed, nHashUsed, vMatch, vnIndex);
 
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1371,7 +1371,7 @@
                 // protocol spec specified allows for us to provide duplicate
                 // txn here, however we MUST always provide at least what the
                 // remote peer needs.
-                typedef std::pair<unsigned int, uint256> PairType;
+                typedef std::pair<size_t, uint256> PairType;
                 for (PairType &pair : merkleBlock.vMatchedTxn) {
                     connman->PushMessage(
                         pfrom, msgMaker.Make(NetMsgType::TX,
diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -382,7 +382,7 @@
     UniValue res(UniValue::VARR);
 
     std::vector<uint256> vMatch;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     if (merkleBlock.txn.ExtractMatches(vMatch, vIndex) !=
         merkleBlock.header.hashMerkleRoot) {
         return res;
diff --git a/src/test/bloom_tests.cpp b/src/test/bloom_tests.cpp
--- a/src/test/bloom_tests.cpp
+++ b/src/test/bloom_tests.cpp
@@ -51,8 +51,9 @@
     std::vector<uint8_t> vch = ParseHex("03614e9b050000000000000001");
     std::vector<char> expected(vch.size());
 
-    for (unsigned int i = 0; i < vch.size(); i++)
+    for (size_t i = 0; i < vch.size(); i++) {
         expected[i] = (char)vch[i];
+    }
 
     BOOST_CHECK_EQUAL_COLLECTIONS(stream.begin(), stream.end(),
                                   expected.begin(), expected.end());
@@ -95,8 +96,9 @@
     std::vector<uint8_t> vch = ParseHex("03ce4299050000000100008001");
     std::vector<char> expected(vch.size());
 
-    for (unsigned int i = 0; i < vch.size(); i++)
+    for (size_t i = 0; i < vch.size(); i++) {
         expected[i] = (char)vch[i];
+    }
 
     BOOST_CHECK_EQUAL_COLLECTIONS(stream.begin(), stream.end(),
                                   expected.begin(), expected.end());
@@ -123,8 +125,9 @@
     std::vector<uint8_t> vch = ParseHex("038fc16b080000000000000001");
     std::vector<char> expected(vch.size());
 
-    for (unsigned int i = 0; i < vch.size(); i++)
+    for (size_t i = 0; i < vch.size(); i++) {
         expected[i] = (char)vch[i];
+    }
 
     BOOST_CHECK_EQUAL_COLLECTIONS(stream.begin(), stream.end(),
                                   expected.begin(), expected.end());
@@ -227,7 +230,7 @@
                                     "ba58f3bdaab65e73b7e9260b"),
                            0);
     {
-        std::vector<uint8_t> data(32 + sizeof(unsigned int));
+        std::vector<uint8_t> data(32 + sizeof(uint32_t));
         memcpy(&data[0], prevOutPoint.GetTxId().begin(), 32);
         uint32_t n = prevOutPoint.GetN();
         memcpy(&data[32], &n, sizeof(uint32_t));
@@ -377,7 +380,7 @@
     BOOST_CHECK(merkleBlock.header.GetHash() == block.GetHash());
 
     BOOST_CHECK(merkleBlock.vMatchedTxn.size() == 1);
-    std::pair<unsigned int, uint256> pair = merkleBlock.vMatchedTxn[0];
+    std::pair<size_t, uint256> pair = merkleBlock.vMatchedTxn[0];
 
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].second ==
                 uint256S("0x74d681e0e03bafa802c8aa084379aa98d9fcd632ddc2ed9782b"
@@ -385,12 +388,13 @@
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].first == 8);
 
     std::vector<uint256> vMatched;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 
     // Also match the 8th transaction
     filter.insert(uint256S(
@@ -410,8 +414,9 @@
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 }
 
 BOOST_AUTO_TEST_CASE(merkle_block_2) {
@@ -475,7 +480,7 @@
     BOOST_CHECK(merkleBlock.header.GetHash() == block.GetHash());
 
     BOOST_CHECK(merkleBlock.vMatchedTxn.size() == 1);
-    std::pair<unsigned int, uint256> pair = merkleBlock.vMatchedTxn[0];
+    std::pair<size_t, uint256> pair = merkleBlock.vMatchedTxn[0];
 
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].second ==
                 uint256S("0xe980fe9f792d014e73b95203dc1335c5f9ce19ac537a419e6df"
@@ -483,12 +488,13 @@
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].first == 0);
 
     std::vector<uint256> vMatched;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 
     // Match an output from the second transaction (the pubkey for address
     // 1DZTzaBHUDM7T3QvUKBz4qXMRpkg8jsfB5)
@@ -524,8 +530,9 @@
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 }
 
 BOOST_AUTO_TEST_CASE(merkle_block_2_with_update_none) {
@@ -589,7 +596,7 @@
     BOOST_CHECK(merkleBlock.header.GetHash() == block.GetHash());
 
     BOOST_CHECK(merkleBlock.vMatchedTxn.size() == 1);
-    std::pair<unsigned int, uint256> pair = merkleBlock.vMatchedTxn[0];
+    std::pair<size_t, uint256> pair = merkleBlock.vMatchedTxn[0];
 
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].second ==
                 uint256S("0xe980fe9f792d014e73b95203dc1335c5f9ce19ac537a419e6df"
@@ -597,12 +604,13 @@
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].first == 0);
 
     std::vector<uint256> vMatched;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 
     // Match an output from the second transaction (the pubkey for address
     // 1DZTzaBHUDM7T3QvUKBz4qXMRpkg8jsfB5)
@@ -634,8 +642,9 @@
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 }
 
 BOOST_AUTO_TEST_CASE(merkle_block_3_and_serialize) {
@@ -671,12 +680,13 @@
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].first == 0);
 
     std::vector<uint256> vMatched;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 
     CDataStream merkleStream(SER_NETWORK, PROTOCOL_VERSION);
     merkleStream << merkleBlock;
@@ -688,8 +698,9 @@
                  "33e3ff84a5c41d9d3febe7c770fdcc96b2c3ff60abe184f19630101");
     std::vector<char> expected(vch.size());
 
-    for (unsigned int i = 0; i < vch.size(); i++)
+    for (size_t i = 0; i < vch.size(); i++) {
         expected[i] = (char)vch[i];
+    }
 
     BOOST_CHECK_EQUAL_COLLECTIONS(expected.begin(), expected.end(),
                                   merkleStream.begin(), merkleStream.end());
@@ -792,7 +803,7 @@
     BOOST_CHECK(merkleBlock.header.GetHash() == block.GetHash());
 
     BOOST_CHECK(merkleBlock.vMatchedTxn.size() == 1);
-    std::pair<unsigned int, uint256> pair = merkleBlock.vMatchedTxn[0];
+    std::pair<size_t, uint256> pair = merkleBlock.vMatchedTxn[0];
 
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].second ==
                 uint256S("0x0a2a92f0bda4727d0a13eaddf4dd9ac6b5c61a1429e6b2b818f"
@@ -800,12 +811,13 @@
     BOOST_CHECK(merkleBlock.vMatchedTxn[0].first == 6);
 
     std::vector<uint256> vMatched;
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 
     // Also match the 4th transaction
     filter.insert(uint256S(
@@ -825,8 +837,9 @@
     BOOST_CHECK(merkleBlock.txn.ExtractMatches(vMatched, vIndex) ==
                 block.hashMerkleRoot);
     BOOST_CHECK(vMatched.size() == merkleBlock.vMatchedTxn.size());
-    for (unsigned int i = 0; i < vMatched.size(); i++)
+    for (size_t i = 0; i < vMatched.size(); i++) {
         BOOST_CHECK(vMatched[i] == merkleBlock.vMatchedTxn[i].second);
+    }
 }
 
 BOOST_AUTO_TEST_CASE(merkle_block_4_test_p2pubkey_only) {
diff --git a/src/test/pmt_tests.cpp b/src/test/pmt_tests.cpp
--- a/src/test/pmt_tests.cpp
+++ b/src/test/pmt_tests.cpp
@@ -87,7 +87,7 @@
 
             // extract merkle root and matched txids from copy
             std::vector<uint256> vMatchTxid2;
-            std::vector<unsigned int> vIndex;
+            std::vector<size_t> vIndex;
             uint256 merkleRoot2 = pmt2.ExtractMatches(vMatchTxid2, vIndex);
 
             // check that it has the same merkle root as the original, and a
@@ -121,7 +121,7 @@
                                 false, false, false, true,  true,  false};
 
     CPartialMerkleTree tree(vTxid, vMatch);
-    std::vector<unsigned int> vIndex;
+    std::vector<size_t> vIndex;
     BOOST_CHECK(tree.ExtractMatches(vTxid, vIndex).IsNull());
 }
 
diff --git a/src/wallet/rpcdump.cpp b/src/wallet/rpcdump.cpp
--- a/src/wallet/rpcdump.cpp
+++ b/src/wallet/rpcdump.cpp
@@ -387,8 +387,8 @@
     // Search partial merkle tree in proof for our transaction and index in
     // valid block
     std::vector<uint256> vMatch;
-    std::vector<unsigned int> vIndex;
-    unsigned int txnIndex = 0;
+    std::vector<size_t> vIndex;
+    size_t txnIndex = 0;
     if (merkleBlock.txn.ExtractMatches(vMatch, vIndex) ==
         merkleBlock.header.hashMerkleRoot) {