diff --git i/Lib/poplib.py w/Lib/poplib.py
index e2b33ef..3283eb6 100644
--- i/Lib/poplib.py
+++ w/Lib/poplib.py
@@ -32,6 +32,12 @@ CR = '\r'
 LF = '\n'
 CRLF = CR+LF
 
+# maximal line length when calling readline(). This is to prevent
+# reading arbitrary lenght lines. RFC 1939 limits POP3 line length to
+# 512 characters, including CRLF. We have selected 2048 just to be on
+# the safe side.
+_MAXLINE = 2048
+
 
 class POP3:
 
@@ -103,7 +109,10 @@ class POP3:
     # Raise error_proto('-ERR EOF') if the connection is closed.
 
     def _getline(self):
-        line = self.file.readline()
+        line = self.file.readline(_MAXLINE + 1)
+        if len(line) > _MAXLINE:
+            raise error_proto('line too long')
+
         if self._debugging > 1: print '*get*', repr(line)
         if not line: raise error_proto('-ERR EOF')
         octets = len(line)
@@ -363,7 +372,10 @@ else:
             line = ""
             renewline = re.compile(r'.*?\n')
             match = renewline.match(self.buffer)
+
             while not match:
+                if len(self.buffer) > _MAXLINE:
+                    raise error_proto('line too long')
                 self._fillBuffer()
                 match = renewline.match(self.buffer)
             line = match.group(0)
diff --git i/Lib/test/test_poplib.py w/Lib/test/test_poplib.py
index 9505c22..4325480 100644
--- i/Lib/test/test_poplib.py
+++ w/Lib/test/test_poplib.py
@@ -81,7 +81,7 @@ class DummyPOP3Handler(asynchat.async_chat):
 
     def cmd_list(self, arg):
         if arg:
-            self.push('+OK %s %s' %(arg, arg))
+            self.push('+OK %s %s' % (arg, arg))
         else:
             self.push('+OK')
             asynchat.async_chat.push(self, LIST_RESP)
@@ -198,6 +198,10 @@ class TestPOP3Class(TestCase):
                     113)
         self.assertEqual(self.client.retr('foo'), expected)
 
+    def test_too_long_lines(self):
+        self.assertRaises(poplib.error_proto, self.client._shortcmd,
+                          'echo %s' % (3000 * 'a'))
+
     def test_dele(self):
         self.assertOK(self.client.dele('foo'))