diff --git a/Lib/cgi.py b/Lib/cgi.py
--- a/Lib/cgi.py
+++ b/Lib/cgi.py
@@ -687,29 +687,70 @@ class FieldStorage:
             self.__write(line)
 
     def read_lines_to_outerboundary(self):
         """Internal: read lines until outerboundary."""
         next = "--" + self.outerboundary
         last = next + "--"
         delim = ""
         last_line_lfend = True
+        last_line = None
+        next_line = None
         while 1:
-            line = self.fp.readline(1<<16)
+            if next_line:
+                line = next_line
+                next_line = None
+            else:
+                # NB - we may interrupt \r\n sequences if they span the 2**16
+                # byte boundary
+                line = self.fp.readline(1 << 16)
+
+            # The previous line ended with \r, so we don't know if it was
+            # really at the end of a line or not until looking at the current
+            # line
+            if last_line:
+                if line[:1] == "\n":
+                    # Oh noes! We actually interrupted a \r\n sequence. Save
+                    # this line for later, and process the previous line again,
+                    # this time with the extra \n added on
+                    next_line = line[1:]
+                    line = last_line + "\n"
+                else:
+                    # There was much rejoicing! This wasn't an interrupted \r\n
+                    # sequence after all, so we can just process the previous line
+                    # as normal
+                    line = last_line
+                    # Save the current line for processing on the next
+                    # iteration
+                    next_line = line
+                last_line = None
+
+            # The previous line was normal...
+            # If the current line ends with \r, it's possible that we've
+            # interrupted a \r\n sequence due to capping the amount of data
+            # received by readline() above.
+            # Save the current line so we can decide after reading the next
+            # line if we've interrupted a \r\n sequence
+            elif line[-1] == "\r":
+                last_line = line
+                continue
+
             if not line:
                 self.done = -1
                 break
+
             if line[:2] == "--" and last_line_lfend:
                 strippedline = line.strip()
                 if strippedline == next:
                     break
                 if strippedline == last:
                     self.done = 1
                     break
             odelim = delim
+
             if line[-2:] == "\r\n":
                 delim = "\r\n"
                 line = line[:-2]
                 last_line_lfend = True
             elif line[-1] == "\n":
                 delim = "\n"
                 line = line[:-1]
                 last_line_lfend = True