diff --git a/commit_tx.c b/commit_tx.c
index 98979cac6..ca29e12c3 100644
--- a/commit_tx.c
+++ b/commit_tx.c
@@ -18,6 +18,7 @@ struct bitcoin_tx *create_commit_tx(const tal_t *ctx,
 	struct bitcoin_tx *tx;
 	const u8 *redeemscript;
 	struct pubkey ourkey, theirkey, to_me;
+	u32 locktime;
 
 	/* Now create commitment tx: one input, two outputs. */
 	tx = bitcoin_tx(ctx, 1, 2);
@@ -32,9 +33,12 @@ struct bitcoin_tx *create_commit_tx(const tal_t *ctx,
 	if (!proto_to_pubkey(theirs->final, &theirkey))
 		return tal_free(tx);
 
+	if (!proto_to_locktime(ours, &locktime))
+		return tal_free(tx);
+
 	/* First output is a P2SH to a complex redeem script (usu. for me) */
 	redeemscript = bitcoin_redeem_revocable(tx, &ourkey,
-						ours->locktime_seconds,
+						locktime,
 						&theirkey,
 						rhash);
 	tx->output[0].script = scriptpubkey_p2sh(tx, redeemscript);
diff --git a/lightning.pb-c.c b/lightning.pb-c.c
index 1970eed12..7e1ab2faf 100644
--- a/lightning.pb-c.c
+++ b/lightning.pb-c.c
@@ -1517,7 +1517,7 @@ const ProtobufCMessageDescriptor anchor__descriptor =
   (ProtobufCMessageInit) anchor__init,
   NULL,NULL,NULL    /* reserved[123] */
 };
-static const ProtobufCFieldDescriptor open_channel__field_descriptors[7] =
+static const ProtobufCFieldDescriptor open_channel__field_descriptors[8] =
 {
   {
     "seed",
@@ -1534,13 +1534,25 @@ static const ProtobufCFieldDescriptor open_channel__field_descriptors[7] =
   {
     "locktime_seconds",
     2,
-    PROTOBUF_C_LABEL_REQUIRED,
+    PROTOBUF_C_LABEL_OPTIONAL,
     PROTOBUF_C_TYPE_UINT32,
-    0,   /* quantifier_offset */
+    offsetof(OpenChannel, locktime_case),
     offsetof(OpenChannel, locktime_seconds),
     NULL,
     NULL,
-    0,             /* flags */
+    0 | PROTOBUF_C_FIELD_FLAG_ONEOF,             /* flags */
+    0,NULL,NULL    /* reserved1,reserved2, etc */
+  },
+  {
+    "locktime_blocks",
+    3,
+    PROTOBUF_C_LABEL_OPTIONAL,
+    PROTOBUF_C_TYPE_UINT32,
+    offsetof(OpenChannel, locktime_case),
+    offsetof(OpenChannel, locktime_blocks),
+    NULL,
+    NULL,
+    0 | PROTOBUF_C_FIELD_FLAG_ONEOF,             /* flags */
     0,NULL,NULL    /* reserved1,reserved2, etc */
   },
   {
@@ -1605,19 +1617,19 @@ static const ProtobufCFieldDescriptor open_channel__field_descriptors[7] =
   },
 };
 static const unsigned open_channel__field_indices_by_name[] = {
-  5,   /* field[5] = anchor */
-  4,   /* field[4] = commitment_fee */
-  3,   /* field[3] = final */
+  6,   /* field[6] = anchor */
+  5,   /* field[5] = commitment_fee */
+  4,   /* field[4] = final */
+  2,   /* field[2] = locktime_blocks */
   1,   /* field[1] = locktime_seconds */
-  2,   /* field[2] = revocation_hash */
+  3,   /* field[3] = revocation_hash */
   0,   /* field[0] = seed */
-  6,   /* field[6] = tx_version */
+  7,   /* field[7] = tx_version */
 };
-static const ProtobufCIntRange open_channel__number_ranges[2 + 1] =
+static const ProtobufCIntRange open_channel__number_ranges[1 + 1] =
 {
   { 1, 0 },
-  { 4, 2 },
-  { 0, 7 }
+  { 0, 8 }
 };
 const ProtobufCMessageDescriptor open_channel__descriptor =
 {
@@ -1627,10 +1639,10 @@ const ProtobufCMessageDescriptor open_channel__descriptor =
   "OpenChannel",
   "",
   sizeof(OpenChannel),
-  7,
+  8,
   open_channel__field_descriptors,
   open_channel__field_indices_by_name,
-  2,  open_channel__number_ranges,
+  1,  open_channel__number_ranges,
   (ProtobufCMessageInit) open_channel__init,
   NULL,NULL,NULL    /* reserved[123] */
 };
diff --git a/lightning.pb-c.h b/lightning.pb-c.h
index 95154e7ea..cb18af640 100644
--- a/lightning.pb-c.h
+++ b/lightning.pb-c.h
@@ -174,6 +174,12 @@ struct  _Anchor
     , 0,NULL, NULL, NULL, 0, 0, 0 }
 
 
+typedef enum {
+  OPEN_CHANNEL__LOCKTIME__NOT_SET = 0,
+  OPEN_CHANNEL__LOCKTIME_LOCKTIME_SECONDS = 2,
+  OPEN_CHANNEL__LOCKTIME_LOCKTIME_BLOCKS = 3,
+} OpenChannel__LocktimeCase;
+
 /*
  * Set channel params.
  */
@@ -184,10 +190,6 @@ struct  _OpenChannel
    * Seed which sets order we create outputs for all transactions.
    */
   uint64_t seed;
-  /*
-   * Relative locktime for outputs going to us.
-   */
-  uint32_t locktime_seconds;
   /*
    * Hash seed for revoking commitment transactions.
    */
@@ -208,10 +210,15 @@ struct  _OpenChannel
    * Maximum transaction version we support.
    */
   uint32_t tx_version;
+  OpenChannel__LocktimeCase locktime_case;
+  union {
+    uint32_t locktime_seconds;
+    uint32_t locktime_blocks;
+  };
 };
 #define OPEN_CHANNEL__INIT \
  { PROTOBUF_C_MESSAGE_INIT (&open_channel__descriptor) \
-    , 0, 0, NULL, NULL, 0, NULL, 0 }
+    , 0, NULL, NULL, 0, NULL, 0, OPEN_CHANNEL__LOCKTIME__NOT_SET, {} }
 
 
 /*
diff --git a/lightning.proto b/lightning.proto
index a2fb9f51c..6c823658a 100644
--- a/lightning.proto
+++ b/lightning.proto
@@ -73,7 +73,10 @@ message open_channel {
   // Seed which sets order we create outputs for all transactions.
   required uint64 seed = 1;
   // Relative locktime for outputs going to us.
-  required uint32 locktime_seconds = 2;
+  oneof locktime {
+    uint32 locktime_seconds = 2;
+    uint32 locktime_blocks = 3;
+  }
   // Hash seed for revoking commitment transactions.
   required sha256_hash revocation_hash = 4;
   // How to pay money to us from commit_tx.
diff --git a/pkt.c b/pkt.c
index 46666c839..16626a039 100644
--- a/pkt.c
+++ b/pkt.c
@@ -45,6 +45,7 @@ struct pkt *openchannel_pkt(const tal_t *ctx,
 	o.final = pubkey_to_proto(ctx, to_me);
 	o.commitment_fee = commitment_fee;
 	o.anchor = anchor;
+	o.locktime_case = OPEN_CHANNEL__LOCKTIME_LOCKTIME_SECONDS;
 	o.locktime_seconds = rel_locktime_seconds;
 	o.tx_version = BITCOIN_TX_VERSION;
 
diff --git a/protobuf_convert.c b/protobuf_convert.c
index 1d86bfc9f..1cf0d1e2a 100644
--- a/protobuf_convert.c
+++ b/protobuf_convert.c
@@ -81,3 +81,18 @@ void proto_to_sha256(const Sha256Hash *pb, struct sha256 *hash)
 	memcpy(hash->u.u8 + 16, &pb->c, 8);
 	memcpy(hash->u.u8 + 24, &pb->d, 8);
 }
+
+bool proto_to_locktime(const OpenChannel *o, uint32_t *locktime)
+{
+	switch (o->locktime_case) {
+	case OPEN_CHANNEL__LOCKTIME_LOCKTIME_SECONDS:
+		*locktime = 500000000 + o->locktime_seconds;
+		break;
+	case OPEN_CHANNEL__LOCKTIME_LOCKTIME_BLOCKS:
+		*locktime = o->locktime_blocks;
+		break;
+	default:
+		return false;
+	}
+	return true;
+}
diff --git a/protobuf_convert.h b/protobuf_convert.h
index be6999b38..f3f8db81e 100644
--- a/protobuf_convert.h
+++ b/protobuf_convert.h
@@ -18,4 +18,6 @@ bool proto_to_pubkey(const BitcoinPubkey *pb, struct pubkey *key);
 struct sha256;
 Sha256Hash *sha256_to_proto(const tal_t *ctx, const struct sha256 *hash);
 void proto_to_sha256(const Sha256Hash *pb, struct sha256 *hash);
+
+bool proto_to_locktime(const OpenChannel *o, uint32_t *locktime);
 #endif /* LIGHTNING_PROTOBUF_CONVERT_H */
diff --git a/test-cli/create-commit-spend-tx.c b/test-cli/create-commit-spend-tx.c
index 197e74d0c..f77bc1d69 100644
--- a/test-cli/create-commit-spend-tx.c
+++ b/test-cli/create-commit-spend-tx.c
@@ -39,6 +39,7 @@ int main(int argc, char *argv[])
 	struct sha256 rhash;
 	size_t p2sh_out;
 	u64 fee = 10000;
+	u32 locktime_seconds;
 
 	err_set_progname(argv[0]);
 
@@ -61,6 +62,8 @@ int main(int argc, char *argv[])
 
 	o1 = pkt_from_file(argv[2], PKT__PKT_OPEN)->open;
 	o2 = pkt_from_file(argv[3], PKT__PKT_OPEN)->open;
+	if (!proto_to_locktime(o2, &locktime_seconds))
+		errx(1, "Invalid locktime in o2");
 
  	/* We need our private key to spend commit output. */
 	privkey = key_from_base58(argv[4], strlen(argv[4]), &testnet, &pubkey1);
@@ -86,7 +89,7 @@ int main(int argc, char *argv[])
 	
 	/* Create redeem script */
 	redeemscript = bitcoin_redeem_revocable(ctx, &pubkey1,
-						o2->locktime_seconds,
+						locktime_seconds,
 						&pubkey2, &rhash);
 
 	/* Now, create transaction to spend it. */
diff --git a/test-cli/create-steal-tx.c b/test-cli/create-steal-tx.c
index 8f3d9cd56..b0e81d777 100644
--- a/test-cli/create-steal-tx.c
+++ b/test-cli/create-steal-tx.c
@@ -36,6 +36,7 @@ int main(int argc, char *argv[])
 	char *tx_hex;
 	EC_KEY *privkey;
 	bool testnet;
+	u32 locktime_seconds;
 
 	err_set_progname(argv[0]);
 
@@ -73,6 +74,8 @@ int main(int argc, char *argv[])
 
 	o1 = pkt_from_file(argv[4], PKT__PKT_OPEN)->open;
 	o2 = pkt_from_file(argv[5], PKT__PKT_OPEN)->open;
+	if (!proto_to_locktime(o2, &locktime_seconds))
+		errx(1, "Invalid locktime in o2");
 
 	if (!pubkey_from_hexstr(argv[6], &outpubkey))
 		errx(1, "Invalid bitcoin pubkey '%s'", argv[6]);
@@ -89,7 +92,7 @@ int main(int argc, char *argv[])
 	/* Now, which commit output?  Match redeem script. */
 	sha256(&revoke_hash, &revoke_preimage, sizeof(revoke_preimage));
 	redeemscript = bitcoin_redeem_revocable(ctx, &pubkey2,
-						o2->locktime_seconds,
+						locktime_seconds,
 						&pubkey1, &revoke_hash);
 	p2sh = scriptpubkey_p2sh(ctx, redeemscript);