diff --git a/src/main.cpp b/src/main.cpp
index 5de4b8c3b..995195289 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -167,13 +167,14 @@ void static ResendWalletTransactions()
 // mapOrphanTransactions
 //
 
-void static AddOrphanTx(const CDataStream& vMsg)
+void AddOrphanTx(const CDataStream& vMsg)
 {
     CTransaction tx;
     CDataStream(vMsg) >> tx;
     uint256 hash = tx.GetHash();
     if (mapOrphanTransactions.count(hash))
         return;
+
     CDataStream* pvMsg = mapOrphanTransactions[hash] = new CDataStream(vMsg);
     BOOST_FOREACH(const CTxIn& txin, tx.vin)
         mapOrphanTransactionsByPrev.insert(make_pair(txin.prevout.hash, pvMsg));
@@ -201,6 +202,23 @@ void static EraseOrphanTx(uint256 hash)
     mapOrphanTransactions.erase(hash);
 }
 
+int LimitOrphanTxSize(int nMaxOrphans)
+{
+    int nEvicted = 0;
+    while (mapOrphanTransactions.size() > nMaxOrphans)
+    {
+        // Evict a random orphan:
+        std::vector<unsigned char> randbytes(32);
+        RAND_bytes(&randbytes[0], 32);
+        uint256 randomhash(randbytes);
+        map<uint256, CDataStream*>::iterator it = mapOrphanTransactions.lower_bound(randomhash);
+        if (it == mapOrphanTransactions.end())
+            it = mapOrphanTransactions.begin();
+        EraseOrphanTx(it->first);
+        ++nEvicted;
+    }
+    return nEvicted;
+}
 
 
 
@@ -2473,6 +2491,11 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
         {
             printf("storing orphan tx %s\n", inv.hash.ToString().substr(0,10).c_str());
             AddOrphanTx(vMsg);
+
+            // DoS prevention: do not allow mapOrphanTransactions to grow unbounded
+            int nEvicted = LimitOrphanTxSize(MAX_ORPHAN_TRANSACTIONS);
+            if (nEvicted > 0)
+                printf("mapOrphan overflow, removed %d tx\n", nEvicted);
         }
         if (tx.nDoS) pfrom->Misbehaving(tx.nDoS);
     }
diff --git a/src/main.h b/src/main.h
index ac6d624e4..25750dae4 100644
--- a/src/main.h
+++ b/src/main.h
@@ -33,6 +33,7 @@ extern const std::string CLIENT_NAME;
 static const unsigned int MAX_BLOCK_SIZE = 1000000;
 static const unsigned int MAX_BLOCK_SIZE_GEN = MAX_BLOCK_SIZE/2;
 static const int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
+static const int MAX_ORPHAN_TRANSACTIONS = MAX_BLOCK_SIZE/100;
 static const int64 COIN = 100000000;
 static const int64 CENT = 1000000;
 static const int64 MIN_TX_FEE = 50000;
diff --git a/src/test/DoS_tests.cpp b/src/test/DoS_tests.cpp
index f29f036eb..0b8941404 100644
--- a/src/test/DoS_tests.cpp
+++ b/src/test/DoS_tests.cpp
@@ -12,6 +12,12 @@
 
 #include <stdint.h>
 
+// Tests this internal-to-main.cpp method:
+extern void AddOrphanTx(const CDataStream& vMsg);
+extern int LimitOrphanTxSize(int nMaxOrphans);
+extern std::map<uint256, CDataStream*> mapOrphanTransactions;
+extern std::multimap<uint256, CDataStream*> mapOrphanTransactionsByPrev;
+
 CService ip(uint32_t i)
 {
     struct in_addr s;
@@ -123,4 +129,77 @@ BOOST_AUTO_TEST_CASE(DoS_checknbits)
     
 }
 
+static uint256 RandomHash()
+{
+    std::vector<unsigned char> randbytes(32);
+    RAND_bytes(&randbytes[0], 32);
+    uint256 randomhash(randbytes);
+    return randomhash;
+}
+
+CTransaction RandomOrphan()
+{
+    std::map<uint256, CDataStream*>::iterator it;
+    it = mapOrphanTransactions.lower_bound(RandomHash());
+    if (it == mapOrphanTransactions.end())
+        it = mapOrphanTransactions.begin();
+    const CDataStream* pvMsg = it->second;
+    CTransaction tx;
+    CDataStream(*pvMsg) >> tx;
+    return tx;
+}
+
+BOOST_AUTO_TEST_CASE(DoS_mapOrphans)
+{
+    CKey key;
+    key.MakeNewKey(true);
+    CBasicKeyStore keystore;
+    keystore.AddKey(key);
+
+    // 50 orphan transactions:
+    for (int i = 0; i < 50; i++)
+    {
+        CTransaction tx;
+        tx.vin.resize(1);
+        tx.vin[0].prevout.n = 0;
+        tx.vin[0].prevout.hash = RandomHash();
+        tx.vin[0].scriptSig << OP_1;
+        tx.vout.resize(1);
+        tx.vout[0].nValue = 1*CENT;
+        tx.vout[0].scriptPubKey.SetBitcoinAddress(key.GetPubKey());
+
+        CDataStream ds;
+        ds << tx;
+        AddOrphanTx(ds);
+    }
+
+    // ... and 50 that depend on other orphans:
+    for (int i = 0; i < 50; i++)
+    {
+        CTransaction txPrev = RandomOrphan();
+
+        CTransaction tx;
+        tx.vin.resize(1);
+        tx.vin[0].prevout.n = 0;
+        tx.vin[0].prevout.hash = txPrev.GetHash();
+        tx.vout.resize(1);
+        tx.vout[0].nValue = 1*CENT;
+        tx.vout[0].scriptPubKey.SetBitcoinAddress(key.GetPubKey());
+        SignSignature(keystore, txPrev, tx, 0);
+
+        CDataStream ds;
+        ds << tx;
+        AddOrphanTx(ds);
+    }
+
+    // Test LimitOrphanTxSize() function:
+    LimitOrphanTxSize(40);
+    BOOST_CHECK(mapOrphanTransactions.size() <= 40);
+    LimitOrphanTxSize(10);
+    BOOST_CHECK(mapOrphanTransactions.size() <= 10);
+    LimitOrphanTxSize(0);
+    BOOST_CHECK(mapOrphanTransactions.empty());
+    BOOST_CHECK(mapOrphanTransactionsByPrev.empty());
+}
+
 BOOST_AUTO_TEST_SUITE_END()