diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 26c5dfe9261b7f..890f4d77cad1cd 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -32,6 +32,8 @@
 import warnings
 import abc
 import signal
+import subprocess
+import textwrap
 import errno
 from itertools import cycle, count
 from collections import deque
@@ -3132,6 +3134,43 @@ def _test_nonblock_pipe_write(self, bufsize):
         self.assertTrue(wf.closed)
         self.assertTrue(rf.closed)
 
+    # bpo-34780
+    @unittest.skipUnless(sys.platform == 'win32', 'Win32-specific test')
+    def test_stdin_init_hang(self):
+        r, w = os.pipe()
+        self.addCleanup(os.close, r)
+        self.addCleanup(os.close, w)
+        code = textwrap.dedent("""
+            import sys
+            sys.stdout.write('OK')
+            sys.stdout.flush()
+            sys.stdin.read()
+        """)
+        p = subprocess.Popen([sys.executable, '-c', code],
+                             stdin=r, stdout=subprocess.PIPE, bufsize=0)
+        try:
+            # Synchronize to increase chances that p is blocked in ReadFile()
+            # when p2 is started.
+            self.assertEqual(p.stdout.read(2), b'OK')
+            p.stdout.close()
+            p2 = subprocess.Popen([sys.executable, '-c', ''], stdin=r)
+            try:
+                start = time.time()
+                while time.time() - start < 10:
+                    exit_code = p2.poll()
+                    if exit_code is not None:
+                        self.assertEqual(exit_code, 0)
+                        break
+                    time.sleep(0.1)
+                else:
+                    self.fail('Child interpreter appears to hang')
+            finally:
+                p2.kill()
+                p2.wait()
+        finally:
+            p.kill()
+            p.wait()
+
 class CMiscIOTest(MiscIOTest):
     io = io
     shutdown_error = "RuntimeError: could not find io module state"
diff --git a/Misc/NEWS.d/next/Windows/2018-09-23-23-02-46.bpo-34780.PIJ3OU.rst b/Misc/NEWS.d/next/Windows/2018-09-23-23-02-46.bpo-34780.PIJ3OU.rst
new file mode 100644
index 00000000000000..1adb8580724c2c
--- /dev/null
+++ b/Misc/NEWS.d/next/Windows/2018-09-23-23-02-46.bpo-34780.PIJ3OU.rst
@@ -0,0 +1,2 @@
+Fix hang on startup if stdin refers to a pipe with an outstanding concurrent
+operation on Windows.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 7a1a6945c102eb..a88d974d34e92f 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6915,7 +6915,8 @@ posix_fdopen(PyObject *self, PyObject *args)
         PyMem_FREE(mode);
         return posix_error();
     }
-#if defined(HAVE_FSTAT) && defined(S_IFDIR) && defined(EISDIR)
+#if !defined(MS_WINDOWS) && defined(HAVE_FSTAT) && defined(S_IFDIR) && \
+    defined(EISDIR)
     {
         struct stat buf;
         const char *msg;
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index b524f09e0a396e..b5a0aec1f84891 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -119,7 +119,8 @@ file_PyObject_Print(PyObject *op, PyFileObject *f, int flags)
 static PyFileObject*
 dircheck(PyFileObject* f)
 {
-#if defined(HAVE_FSTAT) && defined(S_IFDIR) && defined(EISDIR)
+#if !defined(MS_WINDOWS) && defined(HAVE_FSTAT) && defined(S_IFDIR) && \
+    defined(EISDIR)
     struct stat buf;
     int res;
     if (f->f_fp == NULL)