diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index eb7fcbc..4e349a6 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2041,7 +2041,7 @@
 winrestview( {dict})		none	restore view of current window
 winsaveview()			Dict	save view of current window
 winwidth( {nr})			Number	width of window {nr}
-writefile( {list}, {fname} [, {binary}])
+writefile( {list}, {fname} [, {flags}])
 				Number	write list of lines to file {fname}
 xor( {expr}, {expr})		Number  bitwise XOR
 
@@ -6555,14 +6555,20 @@
   :endif
 <
 							*writefile()*
-writefile({list}, {fname} [, {binary}])
+writefile({list}, {fname} [, {flags}])
 		Write |List| {list} to file {fname}.  Each list item is
 		separated with a NL.  Each list item must be a String or
 		Number.
-		When {binary} is equal to "b" binary mode is used: There will
+		When {flags} contains "b" then binary mode is used: There will
 		not be a NL after the last list item.  An empty item at the
 		end does cause the last line in the file to end in a NL.
-		All NL characters are replaced with a NUL character.
+
+		When {flags} contains "a" then append mode is used, lines are
+		append to the file: >
+			:call writefile(["foo"], "event.log", "a")
+			:call writefile(["bar"], "event.log", "a")
+>
+<		All NL characters are replaced with a NUL character.
 		Inserting CR characters needs to be done before passing {list}
 		to writefile().
 		An existing file is overwritten, if possible.
diff --git a/src/Makefile b/src/Makefile
index cfdca44..39e2de9 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1899,8 +1899,12 @@
 	test_insertcount \
 	test_listlbr \
 	test_listlbr_utf8 \
+	test_mapping \
 	test_options \
 	test_qf_title \
+	test_signs \
+	test_utf8 \
+	test_writefile \
 	test10 test11 test12 test13 test14 test15 test16 test17 test18 test19 \
 	test20 test21 test22 test23 test24 test25 test26 test27 test28 test29 \
 	test30 test31 test32 test33 test34 test35 test36 test37 test38 test39 \
diff --git a/src/eval.c b/src/eval.c
index ecc5e3a..ba456f2 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -19689,6 +19689,7 @@
     typval_T	*rettv;
 {
     int		binary = FALSE;
+    int		append = FALSE;
     char_u	*fname;
     FILE	*fd;
     int		ret = 0;
@@ -19704,14 +19705,19 @@
     if (argvars[0].vval.v_list == NULL)
 	return;
 
-    if (argvars[2].v_type != VAR_UNKNOWN
-			      && STRCMP(get_tv_string(&argvars[2]), "b") == 0)
-	binary = TRUE;
+    if (argvars[2].v_type != VAR_UNKNOWN)
+    {
+	if (vim_strchr(get_tv_string(&argvars[2]), 'b') != NULL)
+	    binary = TRUE;
+	if (vim_strchr(get_tv_string(&argvars[2]), 'a') != NULL)
+	    append = TRUE;
+    }
 
     /* Always open the file in binary mode, library functions have a mind of
      * their own about CR-LF conversion. */
     fname = get_tv_string(&argvars[1]);
-    if (*fname == NUL || (fd = mch_fopen((char *)fname, WRITEBIN)) == NULL)
+    if (*fname == NUL || (fd = mch_fopen((char *)fname,
+				      append ? APPENDBIN : WRITEBIN)) == NULL)
     {
 	EMSG2(_(e_notcreate), *fname == NUL ? (char_u *)_("<empty>") : fname);
 	ret = -1;
diff --git a/src/testdir/test_writefile.in b/src/testdir/test_writefile.in
new file mode 100644
index 0000000..f2dc7d5
--- /dev/null
+++ b/src/testdir/test_writefile.in
@@ -0,0 +1,18 @@
+Tests for writefile()
+
+STARTTEST
+:source small.vim
+:%delete _
+:let f = tempname()
+:call writefile(["over","written"], f, "b")
+:call writefile(["hello","world"], f, "b")
+:call writefile(["!", "good"], f, "a")
+:call writefile(["morning"], f, "ab")
+:call writefile(["", "vimmers"], f, "ab")
+:bwipeout!
+:$put =readfile(f)
+:1 delete _
+:w! test.out
+:qa!
+ENDTEST
+
diff --git a/src/testdir/test_writefile.ok b/src/testdir/test_writefile.ok
new file mode 100644
index 0000000..dfb6a2b
--- /dev/null
+++ b/src/testdir/test_writefile.ok
@@ -0,0 +1,5 @@
+hello
+world!
+good
+morning
+vimmers
diff --git a/src/version.c b/src/version.c
index 7bc7153..f9c0ece 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    503,
+/**/
     502,
 /**/
     501,