diff --git a/stdlib/stack.ml b/stdlib/stack.ml
index 4db3d5b43..e65be5f35 100644
--- a/stdlib/stack.ml
+++ b/stdlib/stack.ml
@@ -11,30 +11,30 @@
 (*                                                                     *)
 (***********************************************************************)
 
-type 'a t = { mutable c : 'a list }
+type 'a t = { mutable c : 'a list; mutable len : int; }
 
 exception Empty
 
-let create () = { c = [] }
+let create () = { c = []; len = 0; }
 
-let clear s = s.c <- []
+let clear s = s.c <- []; s.len <- 0
 
-let copy s = { c = s.c }
+let copy s = { c = s.c; len = s.len; }
 
-let push x s = s.c <- x :: s.c
+let push x s = s.c <- x :: s.c; s.len <- s.len + 1
 
 let pop s =
   match s.c with
-    hd::tl -> s.c <- tl; hd
+  | hd::tl -> s.c <- tl; s.len <- s.len - 1; hd
   | []     -> raise Empty
 
 let top s =
   match s.c with
-    hd::_ -> hd
+  | hd::_ -> hd
   | []     -> raise Empty
 
 let is_empty s = (s.c = [])
 
-let length s = List.length s.c
+let length s = s.len
 
 let iter f s = List.iter f s.c
diff --git a/stdlib/stack.mli b/stdlib/stack.mli
index 9b468aa6c..5874e648e 100644
--- a/stdlib/stack.mli
+++ b/stdlib/stack.mli
@@ -47,7 +47,7 @@ val is_empty : 'a t -> bool
 (** Return [true] if the given stack is empty, [false] otherwise. *)
 
 val length : 'a t -> int
-(** Return the number of elements in a stack. *)
+(** Return the number of elements in a stack. Time complexity O(1) *)
 
 val iter : ('a -> unit) -> 'a t -> unit
 (** [iter f s] applies [f] in turn to all elements of [s],
diff --git a/testsuite/tests/lib-stack/Makefile b/testsuite/tests/lib-stack/Makefile
new file mode 100644
index 000000000..299656b24
--- /dev/null
+++ b/testsuite/tests/lib-stack/Makefile
@@ -0,0 +1,15 @@
+#########################################################################
+#                                                                       #
+#                                 OCaml                                 #
+#                                                                       #
+#                 Xavier Clerc, SED, INRIA Rocquencourt                 #
+#                                                                       #
+#   Copyright 2010 Institut National de Recherche en Informatique et    #
+#   en Automatique.  All rights reserved.  This file is distributed     #
+#   under the terms of the Q Public License version 1.0.                #
+#                                                                       #
+#########################################################################
+
+BASEDIR=../..
+include $(BASEDIR)/makefiles/Makefile.several
+include $(BASEDIR)/makefiles/Makefile.common
diff --git a/testsuite/tests/lib-stack/test.ml b/testsuite/tests/lib-stack/test.ml
new file mode 100644
index 000000000..e8df9a264
--- /dev/null
+++ b/testsuite/tests/lib-stack/test.ml
@@ -0,0 +1,127 @@
+(***********************************************************************)
+(*                                                                     *)
+(*                                OCaml                                *)
+(*                                                                     *)
+(*                 Jeremie Dimino, Jane Street Europe                  *)
+(*                                                                     *)
+(*  Copyright 2015 Institut National de Recherche en Informatique et   *)
+(*  en Automatique.  All rights reserved.  This file is distributed    *)
+(*  under the terms of the S Public License version 1.0.               *)
+(*                                                                     *)
+(***********************************************************************)
+
+module S = struct
+  include Stack
+
+  let to_list s = let l = ref [] in iter (fun x -> l := x :: !l) s; !l (* from bottom to top *)
+end
+
+let does_raise f s =
+  try
+    ignore (f s : int);
+    false
+  with S.Empty ->
+    true
+
+let () =
+  let s = S.create () in
+  ();                   assert (S.to_list s = [          ] && S.length s = 0);
+  S.push 1 s;           assert (S.to_list s = [1         ] && S.length s = 1);
+  S.push 2 s;           assert (S.to_list s = [1; 2      ] && S.length s = 2);
+  S.push 3 s;           assert (S.to_list s = [1; 2; 3   ] && S.length s = 3);
+  S.push 4 s;           assert (S.to_list s = [1; 2; 3; 4] && S.length s = 4);
+  assert (S.pop s = 4); assert (S.to_list s = [1; 2; 3;  ] && S.length s = 3);
+  assert (S.pop s = 3); assert (S.to_list s = [1; 2;     ] && S.length s = 2);
+  assert (S.pop s = 2); assert (S.to_list s = [1;        ] && S.length s = 1);
+  assert (S.pop s = 1); assert (S.to_list s = [          ] && S.length s = 0);
+  assert (does_raise S.pop s);
+;;
+
+let () =
+  let s = S.create () in
+  S.push 1 s; assert (S.pop s = 1); assert (does_raise S.pop s);
+  S.push 2 s; assert (S.pop s = 2); assert (does_raise S.pop s);
+  assert (S.length s = 0);
+;;
+
+let () =
+  let s = S.create () in
+  S.push 1 s; assert (S.top s = 1);
+  S.push 2 s; assert (S.top s = 2);
+  S.push 3 s; assert (S.top s = 3);
+  assert (S.top s = 3); assert (S.pop s = 3);
+  assert (S.top s = 2); assert (S.pop s = 2);
+  assert (S.top s = 1); assert (S.pop s = 1);
+  assert (does_raise S.top s);
+  assert (does_raise S.top s);
+;;
+
+let () =
+  let s = S.create () in
+  for i = 1 to 10 do S.push i s done;
+  S.clear s;
+  assert (S.length s = 0);
+  assert (does_raise S.pop s);
+  assert (s = S.create ());
+  S.push 42 s;
+  assert (S.pop s = 42);
+;;
+
+let () =
+  let s1 = S.create () in
+  for i = 1 to 10 do S.push i s1 done;
+  let s2 = S.copy s1 in
+  assert (S.to_list s1 = [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]);
+  assert (S.to_list s2 = [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]);
+  assert (S.length s1 = 10);
+  assert (S.length s2 = 10);
+  for i = 10 downto 1 do
+    assert (S.pop s1 = i);
+  done;
+  for i = 10 downto 1 do
+    assert (S.pop s2 = i);
+  done;
+;;
+
+let () =
+  let s = S.create () in
+  assert (S.is_empty s);
+  for i = 1 to 10 do
+    S.push i s;
+    assert (S.length s = i);
+    assert (not (S.is_empty s));
+  done;
+  for i = 10 downto 1 do
+    assert (S.length s = i);
+    assert (not (S.is_empty s));
+    ignore (S.pop s : int);
+  done;
+  assert (S.length s = 0);
+  assert (S.is_empty s);
+;;
+
+let () =
+  let s = S.create () in
+  for i = 10 downto 1 do S.push i s done;
+  let i = ref 1 in
+  S.iter (fun j -> assert (!i = j); incr i) s;
+;;
+
+let () =
+  let s1 = S.create () in
+  assert (S.length s1 = 0); assert (S.to_list s1 = []);
+  let s2 = S.copy s1 in
+  assert (S.length s1 = 0); assert (S.to_list s1 = []);
+  assert (S.length s2 = 0); assert (S.to_list s2 = []);
+;;
+
+let () =
+  let s1 = S.create () and s2 = S.create () in
+  for i = 1 to 4 do S.push i s1 done;
+  assert (S.length s1 = 4); assert (S.to_list s1 = [1; 2; 3; 4]);
+  let s2 = S.copy s1 in
+  assert (S.length s1 = 4); assert (S.to_list s1 = [1; 2; 3; 4]);
+  assert (S.length s2 = 4); assert (S.to_list s2 = [1; 2; 3; 4]);
+;;
+
+let () = print_endline "OK"
diff --git a/testsuite/tests/lib-stack/test.reference b/testsuite/tests/lib-stack/test.reference
new file mode 100644
index 000000000..d86bac9de
--- /dev/null
+++ b/testsuite/tests/lib-stack/test.reference
@@ -0,0 +1 @@
+OK