diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 489e560..6265258 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -233,6 +233,17 @@
             else:
                 self.fail("1/0 didn't raise an exception")
 
+    # issue 5008
+    def test_append_mode_tell(self):
+        with io.open(support.TESTFN, "wb") as f:
+            f.write(b"xxx")
+        with io.open(support.TESTFN, "ab", buffering=0) as f:
+            self.assertEqual(f.tell(), 3)
+        with io.open(support.TESTFN, "ab") as f:
+            self.assertEqual(f.tell(), 3)
+        with io.open(support.TESTFN, "a") as f:
+            self.assert_(f.tell() > 0)
+
     def test_destructor(self):
         record = []
         class MyFileIO(io.FileIO):
diff --git a/Misc/NEWS b/Misc/NEWS
index 35506cb..b92048c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -137,6 +137,11 @@
 Library
 -------
 
+- Issue #5008: When a file is opened in append mode with the new IO library,
+  do an explicit seek to the end of file (so that e.g. tell() returns the
+  file size rather than 0). This is consistent with the behaviour of the
+  traditional 2.x file object.
+
 - Issue #5013: Fixed a bug in FileHandler which occurred when the delay
   parameter was set.
 
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index 5b840c0..7a71837 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -55,6 +55,9 @@
 
 #define PyFileIO_Check(op) (PyObject_TypeCheck((op), &PyFileIO_Type))
 
+static PyObject *
+portable_lseek(int fd, PyObject *posobj, int whence);
+
 /* Returns 0 on success, -1 with exception set on failure. */
 static int
 internal_close(PyFileIOObject *self)
@@ -315,6 +318,16 @@
 			goto error;
 	}
 
+	if (append) {
+		/* For consistent behaviour, we explicitly seek to the
+		   end of file (otherwise, it might be done only on the
+		   first write()). */
+		PyObject *pos = portable_lseek(self->fd, NULL, 2);
+		if (pos == NULL)
+			goto error;
+		Py_DECREF(pos);
+	}
+
 	goto done;
 
  error: