diff --git a/accept.go b/accept.go
index 65ac2819..98674eec 100644
--- a/accept.go
+++ b/accept.go
@@ -91,6 +91,22 @@ func (b *BlockChain) maybeAcceptBlock(block *btcutil.Block, fastAdd bool) error
 		return RuleError(str)
 	}
 
+	// Find the latest known good checkpoint and prevent blocks which fork
+	// the main chain before it.  This prevents storage of new, otherwise
+	// valid, blocks which build off of old blocks that are likely at a
+	// much easier difficulty and therefore could be used to waste cache and
+	// disk space.
+	checkpointBlock, err := b.findLatestKnownCheckpoint()
+	if err != nil {
+		return err
+	}
+	if checkpointBlock != nil && blockHeight < checkpointBlock.Height() {
+		str := fmt.Sprintf("block at height %d forks the main chain "+
+			"before the previous checkpoint at height %d",
+			blockHeight, checkpointBlock.Height())
+		return RuleError(str)
+	}
+
 	if !fastAdd {
 		// Reject version 1 blocks once a majority of the network has
 		// upgraded.
diff --git a/process.go b/process.go
index 42d08181..281a36c0 100644
--- a/process.go
+++ b/process.go
@@ -119,10 +119,10 @@ func (b *BlockChain) ProcessBlock(block *btcutil.Block, fastAdd bool) error {
 
 	// Find the latest known checkpoint and perform some additional checks
 	// based on the checkpoint.  This provides a few nice properties such as
-	// preventing forks from blocks before the last checkpoint, rejecting
-	// easy to mine, but otherwise bogus, blocks that could be used to eat
-	// memory, and ensuring expected (versus claimed) proof of work
-	// requirements since the last checkpoint are met.
+	// preventing old side chain blocks before the last checkpoint,
+	// rejecting easy to mine, but otherwise bogus, blocks that could be
+	// used to eat memory, and ensuring expected (versus claimed) proof of
+	// work requirements since the last checkpoint are met.
 	blockHeader := &block.MsgBlock().Header
 	checkpointBlock, err := b.findLatestKnownCheckpoint()
 	if err != nil {