diff --git a/basicswap/basicswap.py b/basicswap/basicswap.py
index c3b6554..3ded3a1 100644
--- a/basicswap/basicswap.py
+++ b/basicswap/basicswap.py
@@ -2415,21 +2415,9 @@ class BasicSwap(BaseApp):
             initiate_tx_block_time = int(self.callcoinrpc(coin_from, 'getblock', [initiate_tx_block_hash, ])['time'])
             if offer.lock_type == TxLockTypes.ABS_LOCK_BLOCKS:
                 # Walk the coin_to chain back until block time matches
-                blockchaininfo = self.callcoinrpc(coin_to, 'getblockchaininfo')
-                cblock_hash = blockchaininfo['bestblockhash']
-                cblock_height = blockchaininfo['blocks']
-                max_tries = 1000
-                for i in range(max_tries):
-                    prev_block = self.callcoinrpc(coin_to, 'getblock', [cblock_hash, ])
-                    self.log.debug('prev_block %s', str(prev_block))
-
-                    if prev_block['time'] <= initiate_tx_block_time:
-                        break
-                    # cblock_hash and height are out of step unless loop breaks
-                    cblock_hash = prev_block['previousblockhash']
-                    cblock_height = prev_block['height']
-
-                ensure(prev_block['time'] <= initiate_tx_block_time, 'Block not found for lock height')
+                block_header_at = ci_to.getBlockHeaderAt(initiate_tx_block_time, block_after=True)
+                cblock_hash = block_header_at['hash']
+                cblock_height = block_header_at['height']
 
                 self.log.debug('Setting lock value from height of block %s %s', coin_to, cblock_hash)
                 contract_lock_value = cblock_height + lock_value
@@ -4084,7 +4072,10 @@ class BasicSwap(BaseApp):
             ensure(script_lock_value == expect_sequence, 'sequence mismatch')
         else:
             if offer.lock_type == TxLockTypes.ABS_LOCK_BLOCKS:
-                self.log.warning('TODO: validate absolute lock values')
+                block_header_from = ci_from.getBlockHeaderAt(bid.created_at)
+                chain_height_at_bid_creation = block_header_from['height']
+                ensure(script_lock_value <= chain_height_at_bid_creation + offer.lock_value + atomic_swap_1.ABS_LOCK_BLOCKS_LEEWAY, 'script lock height too high')
+                ensure(script_lock_value >= chain_height_at_bid_creation + offer.lock_value - atomic_swap_1.ABS_LOCK_BLOCKS_LEEWAY, 'script lock height too low')
             else:
                 ensure(script_lock_value <= bid.created_at + offer.lock_value + atomic_swap_1.INITIATE_TX_TIMEOUT, 'script lock time too high')
                 ensure(script_lock_value >= bid.created_at + offer.lock_value, 'script lock time too low')
diff --git a/basicswap/interface/btc.py b/basicswap/interface/btc.py
index eea5ae9..85c06df 100644
--- a/basicswap/interface/btc.py
+++ b/basicswap/interface/btc.py
@@ -243,6 +243,19 @@ class BTCInterface(CoinInterface):
     def getBlockHeader(self, block_hash):
         return self.rpc_callback('getblockheader', [block_hash])
 
+    def getBlockHeaderAt(self, time, block_after=False):
+        blockchaininfo = self.rpc_callback('getblockchaininfo')
+        last_block_header = self.rpc_callback('getblockheader', [blockchaininfo['bestblockhash']])
+
+        max_tries = 5000
+        for i in range(max_tries):
+            prev_block_header = self.rpc_callback('getblock', [last_block_header['previousblockhash']])
+            if prev_block_header['time'] <= time:
+                return last_block_header if block_after else prev_block_header
+
+            last_block_header = prev_block_header
+        raise ValueError(f'Block header not found at time: {time}')
+
     def initialiseWallet(self, key_bytes):
         key_wif = self.encodeKey(key_bytes)
 
diff --git a/basicswap/protocols/atomic_swap_1.py b/basicswap/protocols/atomic_swap_1.py
index 8d9c0a7..166a9ba 100644
--- a/basicswap/protocols/atomic_swap_1.py
+++ b/basicswap/protocols/atomic_swap_1.py
@@ -12,6 +12,7 @@ from basicswap.script import (
 )
 
 INITIATE_TX_TIMEOUT = 40 * 60  # TODO: make variable per coin
+ABS_LOCK_BLOCKS_LEEWAY = 5
 
 
 def buildContractScript(lock_val, secret_hash, pkh_redeem, pkh_refund, op_lock=OpCodes.OP_CHECKSEQUENCEVERIFY):