diff --git a/src/main.cpp b/src/main.cpp
index 01b62bdf60..0865ecc506 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -162,6 +162,29 @@ namespace {
      */
     map<uint256, NodeId> mapBlockSource;
 
+    /**
+     * Filter for transactions that were recently rejected by
+     * AcceptToMemoryPool. These are not rerequested until the chain tip
+     * changes, at which point the entire filter is reset. Protected by
+     * cs_main.
+     *
+     * Without this filter we'd be re-requesting txs from each of our peers,
+     * increasing bandwidth consumption considerably. For instance, with 100
+     * peers, half of which relay a tx we don't accept, that might be a 50x
+     * bandwidth increase. A flooding attacker attempting to roll-over the
+     * filter using minimum-sized, 60byte, transactions might manage to send
+     * 1000/sec if we have fast peers, so we pick 120,000 to give our peers a
+     * two minute window to send invs to us.
+     *
+     * Decreasing the false positive rate is fairly cheap, so we pick one in a
+     * million to make it highly unlikely for users to have issues with this
+     * filter.
+     *
+     * Memory used: 1.7MB
+     */
+    boost::scoped_ptr<CRollingBloomFilter> recentRejects;
+    uint256 hashRecentRejectsChainTip;
+
     /** Blocks that are in flight, and that are in the queue to be downloaded. Protected by cs_main. */
     struct QueuedBlock {
         uint256 hash;
@@ -3267,6 +3290,7 @@ void UnloadBlockIndex()
     setDirtyBlockIndex.clear();
     setDirtyFileInfo.clear();
     mapNodeState.clear();
+    recentRejects.reset(NULL);
 
     BOOST_FOREACH(BlockMap::value_type& entry, mapBlockIndex) {
         delete entry.second;
@@ -3320,6 +3344,9 @@ bool InitBlockIndex() {
         }
     }
 
+    // Initialize global variables that cannot be constructed at startup.
+    recentRejects.reset(new CRollingBloomFilter(120000, 0.000001));
+
     return true;
 }
 
@@ -3689,10 +3716,20 @@ bool static AlreadyHave(const CInv& inv) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
     {
     case MSG_TX:
         {
-            bool txInMap = false;
-            txInMap = mempool.exists(inv.hash);
-            return txInMap || mapOrphanTransactions.count(inv.hash) ||
-                pcoinsTip->HaveCoins(inv.hash);
+            if (chainActive.Tip()->GetBlockHash() != hashRecentRejectsChainTip)
+            {
+                // If the chain tip has changed previously rejected transactions
+                // might be now valid, e.g. due to a nLockTime'd tx becoming valid,
+                // or a double-spend. Reset the rejects filter and give those
+                // txs a second chance.
+                hashRecentRejectsChainTip = chainActive.Tip()->GetBlockHash();
+                recentRejects->reset();
+            }
+
+            return recentRejects->contains(inv.hash) ||
+                   mempool.exists(inv.hash) ||
+                   mapOrphanTransactions.count(inv.hash) ||
+                   pcoinsTip->HaveCoins(inv.hash);
         }
     case MSG_BLOCK:
         return mapBlockIndex.count(inv.hash);
@@ -4292,6 +4329,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                         // Probably non-standard or insufficient fee/priority
                         LogPrint("mempool", "   removed orphan tx %s\n", orphanHash.ToString());
                         vEraseQueue.push_back(orphanHash);
+                        recentRejects->insert(orphanHash);
                     }
                     mempool.check(pcoinsTip);
                 }
@@ -4309,11 +4347,23 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             unsigned int nEvicted = LimitOrphanTxSize(nMaxOrphanTx);
             if (nEvicted > 0)
                 LogPrint("mempool", "mapOrphan overflow, removed %u tx\n", nEvicted);
-        } else if (pfrom->fWhitelisted) {
-            // Always relay transactions received from whitelisted peers, even
-            // if they are already in the mempool (allowing the node to function
-            // as a gateway for nodes hidden behind it).
-            RelayTransaction(tx);
+        } else {
+            // AcceptToMemoryPool() returned false, possibly because the tx is
+            // already in the mempool; if the tx isn't in the mempool that
+            // means it was rejected and we shouldn't ask for it again.
+            if (!mempool.exists(tx.GetHash())) {
+                recentRejects->insert(tx.GetHash());
+            }
+            if (pfrom->fWhitelisted) {
+                // Always relay transactions received from whitelisted peers, even
+                // if they were rejected from the mempool, allowing the node to
+                // function as a gateway for nodes hidden behind it.
+                //
+                // FIXME: This includes invalid transactions, which means a
+                // whitelisted peer could get us banned! We may want to change
+                // that.
+                RelayTransaction(tx);
+            }
         }
         int nDoS = 0;
         if (state.IsInvalid(nDoS))