diff --git a/src/compatibility.cpp b/src/compatibility.cpp
index 870d2ede6d..ca5d8293c3 100644
--- a/src/compatibility.cpp
+++ b/src/compatibility.cpp
@@ -111,6 +111,7 @@ static FCompatOption Options[] =
 	{ "linkfrozenprops",		BCOMPATF_LINKFROZENPROPS, SLOT_BCOMPAT },
 	{ "disablepushwindowcheck",	BCOMPATF_NOWINDOWCHECK, SLOT_BCOMPAT },
 	{ "floatbob",				BCOMPATF_FLOATBOB, SLOT_BCOMPAT },
+	{ "noslopeid",				BCOMPATF_NOSLOPEID, SLOT_BCOMPAT },
 
 	// list copied from g_mapinfo.cpp
 	{ "shorttex",				COMPATF_SHORTTEX, SLOT_COMPAT },
diff --git a/src/doomdef.h b/src/doomdef.h
index 9ebe45cf7f..a36a7a1c2d 100644
--- a/src/doomdef.h
+++ b/src/doomdef.h
@@ -358,6 +358,7 @@ enum
 	BCOMPATF_LINKFROZENPROPS	= 1 << 6,	// Clearing PROP_TOTALLYFROZEN or PROP_FROZEN also clears the other
 	BCOMPATF_NOWINDOWCHECK		= 1 << 7,	// Disable the window check in CheckForPushSpecial()
 	BCOMPATF_FLOATBOB			= 1 << 8,	// Use Hexen's original method of preventing floatbobbing items from falling down
+	BCOMPATF_NOSLOPEID			= 1 << 9,	// disable line IDs on slopes.
 };
 
 // phares 3/20/98:
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index 8a69c2de20..92636fb0cd 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -1959,7 +1959,7 @@ void P_SetLineID (int i, line_t *ld)
 			break;
 			
 		case Plane_Align:
-			setid = ld->args[2];
+			if (!(ib_compatflags & BCOMPATF_NOSLOPEID)) setid = ld->args[2];
 			break;
 			
 		case Static_Init: