diff --git a/stew/io2.nim b/stew/io2.nim
index ccc222b..2e54d6a 100644
--- a/stew/io2.nim
+++ b/stew/io2.nim
@@ -32,6 +32,7 @@ when defined(windows):
     FILE_FLAG_OVERLAPPED = 0x40000000'u32
     FILE_SHARE_READ = 0x00000001'u32
     FILE_SHARE_WRITE = 0x00000002'u32
+    FILE_APPEND_DATA = 0x00000004'u32
 
     FILE_FLAG_NO_BUFFERING = 0x20000000'u32
     FILE_ATTRIBUTE_READONLY = 0x00000001'u32
@@ -1012,7 +1013,14 @@ proc openFile*(pathName: string, flags: set[OpenFlags],
     if OpenFlags.Truncate in flags:
       cflags = cflags or posix.O_TRUNC
     if OpenFlags.Append in flags:
-      cflags = cflags or posix.O_APPEND
+      if ((cflags and posix.O_WRONLY) != 0) or ((cflags and posix.O_RDWR) != 0):
+        cflags = cflags or posix.O_APPEND
+      else:
+        if ((cflags and posix.O_RDONLY) != 0):
+          cflags = cflags and not(posix.O_RDONLY)
+          cflags = cflags or posix.O_APPEND or O_RDWR
+        else:
+          cflags = cflags or posix.O_APPEND or O_WRONLY
     when defined(linux) or defined(freebsd) or defined(netbsd) or
          defined(dragonflybsd):
       if OpenFlags.Direct in flags:
@@ -1053,7 +1061,7 @@ proc openFile*(pathName: string, flags: set[OpenFlags],
     var
       dwAccess: uint32
       dwShareMode: uint32
-      dwCreation: uint32
+      dwCreation: uint32 = OPEN_EXISTING
       dwFlags: uint32
 
     var sa = SECURITY_ATTRIBUTES(
@@ -1062,36 +1070,37 @@ proc openFile*(pathName: string, flags: set[OpenFlags],
       bInheritHandle: 0
     )
 
-    if (OpenFlags.Write in flags) and (OpenFlags.Read in flags):
-      dwAccess = dwAccess or (GENERIC_READ or GENERIC_WRITE)
-    else:
-      if OpenFlags.Write in flags:
-        dwAccess = dwAccess or GENERIC_WRITE
-      else:
-        dwAccess = dwAccess or GENERIC_READ
+    if OpenFlags.Write in flags:
+      dwAccess = dwAccess or GENERIC_WRITE
+    if OpenFlags.Read in flags:
+      dwAccess = dwAccess or GENERIC_READ
+    if OpenFlags.Append in flags:
+      dwAccess = dwAccess or FILE_APPEND_DATA
 
-    if {OpenFlags.Create, OpenFlags.Exclusive} <= flags:
-      dwCreation = dwCreation or CREATE_NEW
-    elif OpenFlags.Truncate in flags:
+    if OpenFlags.Truncate in flags:
       if OpenFlags.Create in flags:
         dwCreation = dwCreation or CREATE_ALWAYS
-      elif OpenFlags.Read notin flags:
+      else:
         dwCreation = dwCreation or TRUNCATE_EXISTING
-    elif OpenFlags.Append in flags:
-      dwCreation = dwCreation or OPEN_EXISTING
-    elif OpenFlags.Create in flags:
-      dwCreation = dwCreation or OPEN_ALWAYS
+
+    if {OpenFlags.Create} == flags or
+       {OpenFlags.Create, OpenFlags.Exclusive} == flags:
+      dwCreation = dwCreation or CREATE_NEW
     else:
-      dwCreation = dwCreation or OPEN_EXISTING
+      if OpenFlags.Create in flags:
+        dwCreation = dwCreation and not(OPEN_EXISTING)
+        dwCreation = dwCreation or OPEN_ALWAYS
 
     if dwCreation == OPEN_EXISTING and
-       ((dwAccess and (GENERIC_READ or GENERIC_WRITE)) == GENERIC_READ):
-      dwShareMode = dwShareMode or FILE_SHARE_READ
+       ((dwAccess and GENERIC_READ) == GENERIC_READ):
+      if OpenFlags.Exclusive notin flags:
+        dwShareMode = dwShareMode or FILE_SHARE_READ
 
-    if OpenFlags.ShareRead in flags:
-      dwShareMode = dwShareMode or FILE_SHARE_READ
-    if OpenFlags.ShareWrite in flags:
-      dwShareMode = dwShareMode or FILE_SHARE_WRITE
+    if OpenFlags.Exclusive notin flags:
+      if OpenFlags.ShareRead in flags:
+        dwShareMode = dwShareMode or FILE_SHARE_READ
+      if OpenFlags.ShareWrite in flags:
+        dwShareMode = dwShareMode or FILE_SHARE_WRITE
 
     if OpenFlags.NonBlock in flags:
       dwFlags = dwFlags or FILE_FLAG_OVERLAPPED
diff --git a/tests/test_io2.nim b/tests/test_io2.nim
index 59a93fc..83af09d 100644
--- a/tests/test_io2.nim
+++ b/tests/test_io2.nim
@@ -406,35 +406,136 @@ suite "OS Input/Output procedures test suite":
       removeFile("testblob6").isOk()
 
   test "openFile()/readFile()/writeFile() test":
-    var buffer = newString(10)
-    let flags = {OpenFlags.Write, OpenFlags.Truncate, OpenFlags.Create}
+    var buffer = newString(100)
 
-    var fdres = openFile("testfile.txt", flags)
-    check:
-      fdres.isOk()
-      readFile(fdres.get(), buffer).isErr()
-      writeFile(fdres.get(), "TEST").isOk()
-      readFile(fdres.get(), buffer).isErr()
-      closeFile(fdres.get()).isOk()
+    block:
+      let
+        flags = {OpenFlags.Write, OpenFlags.Truncate, OpenFlags.Create}
+        fdres = openFile("testfile.txt", flags)
+      check:
+        fdres.isOk()
+        readFile(fdres.get(), buffer).isErr()
+        writeFile(fdres.get(), "TEST1").isOk()
+        readFile(fdres.get(), buffer).isErr()
+        closeFile(fdres.get()).isOk()
 
-    fdres = openFile("testfile.txt", {OpenFlags.Read})
-    check:
-      fdres.isOk()
-      readFile(fdres.get(), buffer).isOk()
-      writeFile(fdres.get(), "TEST2").isErr()
-      readFile(fdres.get(), buffer).isOk()
-      closeFile(fdres.get()).isOk()
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read})
+      check fdres.isOk()
+      let res1 = readFile(fdres.get(), buffer)
+      check:
+        res1.isOk()
+        res1.get() == uint(5)
+        writeFile(fdres.get(), "TEST2").isErr()
+      let res2 = readFile(fdres.get(), buffer)
+      check:
+        res2.isOk()
+        res2.get() == uint(0)
+        closeFile(fdres.get()).isOk()
 
-    fdres = openFile("testfile.txt", {OpenFlags.Read, OpenFlags.Write})
-    check:
-      fdres.isOk()
-      readFile(fdres.get(), buffer).isOk()
-      writeFile(fdres.get(), "TEST2").isOk()
-      closeFile(fdres.get()).isOk()
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read, OpenFlags.Write})
+      check fdres.isOk()
+      let res1 = readFile(fdres.get(), buffer)
+      check:
+        res1.isOk()
+        res1.get() == uint(5)
+        writeFile(fdres.get(), "TEST3").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read})
+      check fdres.isOk()
+      let res1 = readFile(fdres.get(), buffer)
+      check:
+        res1.isOk()
+        res1.get() == uint(10)
+        buffer[0 .. 9] == "TEST1TEST3"
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Write})
+      check:
+        fdres.isOk()
+        writeFile(fdres.get(), "TEST2").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read})
+      check fdres.isOk()
+      let res1 = readFile(fdres.get(), buffer)
+      check:
+        res1.isOk()
+        res1.get() == uint(10)
+        buffer[0 .. 9] == "TEST2TEST3"
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Append})
+      check:
+        fdres.isOk()
+        writeFile(fdres.get(), "TEST4").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Append})
+      check:
+        fdres.isOk()
+        writeFile(fdres.get(), "TEST5").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read})
+      check fdres.isOk()
+      let res = readFile(fdres.get(), buffer)
+      check:
+        res.isOk()
+        res.get() == 20
+        buffer[0 .. 19] == "TEST2TEST3TEST4TEST5"
+        closeFile(fdres.get()).isOk()
 
     check:
       removeFile("testfile.txt").isOk()
 
+    # Create file if not exists and append data to file.
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Append, OpenFlags.Create})
+      check:
+        fdres.isOk()
+        writeFile(fdres.get(), "TEST1").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Append, OpenFlags.Create})
+      check:
+        fdres.isOk()
+        writeFile(fdres.get(), "TEST2").isOk()
+        closeFile(fdres.get()).isOk()
+
+    block:
+      let fdres = openFile("testfile.txt", {OpenFlags.Read})
+      check fdres.isOk()
+      let res = readFile(fdres.get(), buffer)
+      check:
+        res.isOk()
+        res.get() == 10
+        buffer[0 .. 9] == "TEST1TEST2"
+        closeFile(fdres.get()).isOk()
+
+    check:
+      removeFile("testfile.txt").isOk()
+
+    # Open nonexisting file with different flags.
+
+    check:
+      openFile("testfile.txt", {OpenFlags.Append}).isErr()
+      openFile("testfile.txt", {OpenFlags.Read}).isErr()
+      openFile("testfile.txt", {OpenFlags.Write}).isErr()
+      openFile("testfile.txt", {OpenFlags.Read, OpenFlags.Write}).isErr()
+      openFile("testfile.txt", {OpenFlags.Read, OpenFlags.Write,
+                                OpenFlags.Append}).isErr()
+
   test "toString(set[Permission]) test":
     let emptyMask: set[Permission] = {}
     check: