diff --git a/gitk b/gitk
index 628f6e5c5b..bdd12236b8 100755
--- a/gitk
+++ b/gitk
@@ -2362,6 +2362,7 @@ proc makewindow {} {
 	{mc "Mark this commit" command markhere}
 	{mc "Return to mark" command gotomark}
 	{mc "Find descendant of this and mark" command find_common_desc}
+	{mc "Compare with marked commit" command compare_commits}
     }
     $rowctxmenu configure -tearoff 0
 
@@ -8037,9 +8038,11 @@ proc rowmenu {x y id} {
 	if {[info exists markedid] && $markedid ne $id} {
 	    $menu entryconfigure 9 -state normal
 	    $menu entryconfigure 10 -state normal
+	    $menu entryconfigure 11 -state normal
 	} else {
 	    $menu entryconfigure 9 -state disabled
 	    $menu entryconfigure 10 -state disabled
+	    $menu entryconfigure 11 -state disabled
 	}
     } else {
 	set menu $fakerowmenu
@@ -8103,6 +8106,92 @@ proc find_common_desc {} {
     #puts "took [expr {$t2-$t1}]ms"
 }
 
+proc compare_commits {} {
+    global markedid rowmenuid curview children
+
+    if {![info exists markedid]} return
+    if {![commitinview $markedid $curview]} return
+    addtohistory [list do_cmp_commits $markedid $rowmenuid]
+    do_cmp_commits $markedid $rowmenuid
+}
+
+proc getpatchid {id} {
+    global patchids
+
+    if {![info exists patchids($id)]} {
+	set x [exec git diff-tree -p --root $id | git patch-id]
+	set patchids($id) [lindex $x 0]
+    }
+    return $patchids($id)
+}
+
+proc do_cmp_commits {a b} {
+    global ctext curview parents children patchids commitinfo
+
+    $ctext conf -state normal
+    clear_ctext
+    init_flist {}
+    for {set i 0} {$i < 100} {incr i} {
+	set shorta [string range $a 0 7]
+	set shortb [string range $b 0 7]
+	set skipa 0
+	set skipb 0
+	if {[llength $parents($curview,$a)] > 1} {
+	    appendwithlinks [mc "Skipping merge commit %s\n" $shorta] {}
+	    set skipa 1
+	} else {
+	    set patcha [getpatchid $a]
+	}
+	if {[llength $parents($curview,$b)] > 1} {
+	    appendwithlinks [mc "Skipping merge commit %s\n" $shortb] {}
+	    set skipb 1
+	} else {
+	    set patchb [getpatchid $b]
+	}
+	if {!$skipa && !$skipb} {
+	    set heada [lindex $commitinfo($a) 0]
+	    set headb [lindex $commitinfo($b) 0]
+	    if {$patcha eq $patchb} {
+		if {$heada eq $headb} {
+		    appendwithlinks [mc "Commit %s == %s  %s\n" \
+					 $shorta $shortb $heada] {}
+		} else {
+		    appendwithlinks [mc "Commit %s  %s\n" $shorta $heada] {}
+		    appendwithlinks [mc " is the same patch as\n"] {}
+		    appendwithlinks [mc "       %s  %s\n" $shortb $headb] {}
+		}
+		set skipa 1
+		set skipb 1
+	    } else {
+		$ctext insert end "\n"
+		appendwithlinks [mc "Commit %s  %s\n" $shorta $heada] {}
+		appendwithlinks [mc " differs from\n"] {}
+		appendwithlinks [mc "       %s  %s\n" $shortb $headb] {}
+		appendwithlinks [mc "- stopping\n"]
+		break
+	    }
+	}
+	if {$skipa} {
+	    if {[llength $children($curview,$a)] != 1} {
+		$ctext insert end "\n"
+		appendwithlinks [mc "Commit %s has %s children - stopping\n" \
+				    $shorta [llength $children($curview,$a)]] {}
+		break
+	    }
+	    set a [lindex $children($curview,$a) 0]
+	}
+	if {$skipb} {
+	    if {[llength $children($curview,$b)] != 1} {
+		appendwithlinks [mc "Commit %s has %s children - stopping\n" \
+				    $shortb [llength $children($curview,$b)]] {}
+		break
+	    }
+	    set b [lindex $children($curview,$b) 0]
+	}
+    }
+    $ctext conf -state disabled
+}
+
 proc diffvssel {dirn} {
     global rowmenuid selectedline