diff --git a/Lib/codecs.py b/Lib/codecs.py
index 061bc1cdffc..92c6feff31f 100644
--- a/Lib/codecs.py
+++ b/Lib/codecs.py
@@ -303,14 +303,11 @@ def readlines(self, sizehint=None):
             Line breaks are implemented using the codec's decoder
             method and are included in the list entries.
 
-            sizehint, if given, is passed as size argument to the
-            stream's .read() method.
+            sizehint, if given, is ignored since there is no efficient
+            way to finding the true end-of-line.
 
         """
-        if sizehint is None:
-            data = self.stream.read()
-        else:
-            data = self.stream.read(sizehint)
+        data = self.stream.read()
         return self.decode(data, self.errors)[0].splitlines(1)
 
     def reset(self):
@@ -488,10 +485,7 @@ def readline(self, size=None):
 
     def readlines(self, sizehint=None):
 
-        if sizehint is None:
-            data = self.reader.read()
-        else:
-            data = self.reader.read(sizehint)
+        data = self.reader.read()
         data, bytesencoded = self.encode(data, self.errors)
         return data.splitlines(1)