diff --git a/sys/netinet/tcp_lro.h b/sys/netinet/tcp_lro.h
--- a/sys/netinet/tcp_lro.h
+++ b/sys/netinet/tcp_lro.h
@@ -146,8 +146,9 @@
 	uint16_t		compressed;
 	uint16_t		uncompressed;
 	uint16_t		window;
-	uint16_t		timestamp : 1;
-	uint16_t		needs_merge : 1;
+	uint8_t			flags;
+	uint8_t			timestamp : 1;
+	uint8_t			needs_merge : 1;
 	struct bintime		alloc_time;	/* time when entry was allocated */
 };
 
diff --git a/sys/netinet/tcp_lro.c b/sys/netinet/tcp_lro.c
--- a/sys/netinet/tcp_lro.c
+++ b/sys/netinet/tcp_lro.c
@@ -921,6 +921,7 @@
 	le->next_seq = ntohl(th->th_seq) + tcp_data_len;
 	le->ack_seq = th->th_ack;
 	le->window = th->th_win;
+	le->flags = th->th_flags;
 	le->needs_merge = 0;
 
 	/* Setup new data pointers. */
@@ -1092,10 +1093,12 @@
 		}
 		/* Try to append the new segment. */
 		if (__predict_false(ntohl(th->th_seq) != le->next_seq ||
+				    ((th->th_flags & TH_ACK) !=
+				      (le->flags & TH_ACK)) ||
 				    (tcp_data_len == 0 &&
 				     le->ack_seq == th->th_ack &&
 				     le->window == th->th_win))) {
-			/* Out of order packet or duplicate ACK. */
+			/* Out of order packet, non-ACK + ACK or dup ACK. */
 			tcp_push_and_replace(lc, le, m);
 			goto again;
 		}