diff --git a/libethereum/BlockChain.cpp b/libethereum/BlockChain.cpp
index a5401dbbf..886dbd2aa 100644
--- a/libethereum/BlockChain.cpp
+++ b/libethereum/BlockChain.cpp
@@ -42,12 +42,16 @@ BlockChain::~BlockChain()
 {
 }
 
-u256s BlockChain::blockChain() const
+u256s BlockChain::blockChain(u256Set const& _earlyExit) const
 {
 	// TODO: return the current valid block chain from most recent to genesis.
 	// TODO: arguments for specifying a set of early-ends
 	u256s ret;
-
+	ret.reserve(m_numberAndParent[m_lastBlockHash].first + 1);
+	auto i = m_lastBlockHash;
+	for (; i != m_genesisHash && !_earlyExit.count(i); i = m_numberAndParent[i].second)
+		ret.push_back(i);
+	ret.push_back(i);
 	return ret;
 }
 
@@ -73,13 +77,9 @@ void BlockChain::import(bytes const& _block)
 			return;
 		bi.number = it->second.first + 1;
 
-		// Check Ancestry:
-		// Check timestamp is after previous timestamp.
-		if (bi.timestamp <= BlockInfo(block(bi.parentHash)).timestamp)
-			throw InvalidTimestamp();
-
-		// TODO: check difficulty is correct given the two timestamps.
-//		if (bi.timestamp )
+		// Check family:
+		BlockInfo bip(block(bi.parentHash));
+		bi.verifyParent(bip);
 
 		// TODO: check transactions are valid and that they result in a state equivalent to our state_root.
 		// this saves us from an embarrassing exit later.
@@ -95,13 +95,14 @@ void BlockChain::import(bytes const& _block)
 		}
 
 		// Insert into DB
-		m_numberAndParent[newHash] = make_pair(bi.number, bi.parentHash);
+		m_numberAndParent[newHash] = make_pair((uint)bi.number, bi.parentHash);
 		m_children.insert(make_pair(bi.parentHash, newHash));
 		ldb::WriteOptions o;
 		m_db->Put(o, ldb::Slice(toBigEndianString(newHash)), (ldb::Slice)ref(_block));
 
 		// This might be the new last block; count back through ancestors to common shared ancestor and compare to current.
 		// TODO: Use GHOST algorithm.
+
 	}
 	catch (...)
 	{
diff --git a/libethereum/BlockChain.h b/libethereum/BlockChain.h
index f85708a44..a0ebb7946 100644
--- a/libethereum/BlockChain.h
+++ b/libethereum/BlockChain.h
@@ -51,7 +51,7 @@ public:
 	bytesConstRef lastBlock() const { return block(m_lastBlockHash); }
 
 	/// Get the full block chain, according to the GHOST algo and the blocks available in the db.
-	u256s blockChain() const;
+	u256s blockChain(u256Set const& _earlyExit) const;
 
 	/// Get the number of the last block of the longest chain.
 	u256 lastBlockNumber() const;
@@ -60,7 +60,7 @@ public:
 
 private:
 	/// Get fully populated from disk DB.
-	mutable std::map<u256, std::pair<u256, u256>> m_numberAndParent;
+	mutable std::map<u256, std::pair<uint, u256>> m_numberAndParent;
 	mutable std::multimap<u256, u256> m_children;
 
 	ldb::DB* m_db;
diff --git a/libethereum/BlockInfo.cpp b/libethereum/BlockInfo.cpp
index bb3b162b0..ffaac5e75 100644
--- a/libethereum/BlockInfo.cpp
+++ b/libethereum/BlockInfo.cpp
@@ -84,7 +84,7 @@ void BlockInfo::populate(bytesConstRef _block, u256 _number)
 	}
 }
 
-void BlockInfo::verifyInternals(bytesConstRef _block)
+void BlockInfo::verifyInternals(bytesConstRef _block) const
 {
 	RLP root(_block);
 
@@ -99,3 +99,32 @@ void BlockInfo::verifyInternals(bytesConstRef _block)
 	if (d.eval(nonce) >= difficulty)
 		throw InvalidNonce();
 }
+
+u256 BlockInfo::calculateDifficulty(BlockInfo const& _parent) const
+{
+	/*
+	D(genesis_block) = 2^36
+	D(block) =
+		if block.timestamp >= block.parent.timestamp + 42: D(block.parent) - floor(D(block.parent) / 1024)
+		else:                                              D(block.parent) + floor(D(block.parent) / 1024)
+			*/
+	if (number == 0)
+		return (u256)1 << 36;
+	else
+		return timestamp >= _parent.timestamp + 42 ? _parent.difficulty - (_parent.difficulty >> 10) : (_parent.difficulty + (_parent.difficulty >> 10));
+}
+
+void BlockInfo::verifyParent(BlockInfo const& _parent) const
+{
+	if (number == 0)
+		// Genesis block - no parent.
+		return;
+
+	// Check timestamp is after previous timestamp.
+	if (_parent.timestamp <= _parent.timestamp)
+		throw InvalidTimestamp();
+
+	// Check difficulty is correct given the two timestamps.
+	if (difficulty != calculateDifficulty(_parent))
+		throw InvalidDifficulty();
+}
diff --git a/libethereum/BlockInfo.h b/libethereum/BlockInfo.h
index 96f72be5e..5163b50f9 100644
--- a/libethereum/BlockInfo.h
+++ b/libethereum/BlockInfo.h
@@ -49,7 +49,10 @@ public:
 
 	static BlockInfo const& genesis() { if (!s_genesis) (s_genesis = new BlockInfo)->populateGenesis(); return *s_genesis; }
 	void populate(bytesConstRef _block, u256 _number = 0);
-	void verifyInternals(bytesConstRef _block);
+	void verifyInternals(bytesConstRef _block) const;
+	void verifyParent(BlockInfo const& _parent) const;
+
+	u256 calculateDifficulty(BlockInfo const& _bi) const;
 
 	/// No-nonce sha3 of the header only.
 	u256 headerHashWithoutNonce() const;
diff --git a/libethereum/State.cpp b/libethereum/State.cpp
index 2be0bb6e7..e7a99635b 100644
--- a/libethereum/State.cpp
+++ b/libethereum/State.cpp
@@ -83,7 +83,7 @@ void State::sync(BlockChain const& _bc, TransactionQueue& _tq)
 		// New blocks available, or we've switched to a different branch. All change.
 		// TODO: Find most recent state dump and replay what's left.
 		// (Most recent state dump might end up being genesis.)
-		std::vector<u256> l = _bc.blockChain();	// TODO: take u256Set "restorePoints" argument so it needs only give us the chain up until some restore point in the past where we stored the state.
+		std::vector<u256> l = _bc.blockChain(u256Set());	// TODO: take u256Set "restorePoints" argument so it needs only give us the chain up until some restore point in the past where we stored the state.
 
 		if (l.back() == BlockInfo::genesis().hash)
 		{
@@ -125,6 +125,8 @@ void State::sync(BlockChain const& _bc, TransactionQueue& _tq)
 			}
 			catch (...)
 			{
+				// Something else went wrong - drop it.
+				_tq.drop(i.first);
 			}
 }