diff --git a/setuptools/dist.py b/setuptools/dist.py
index 05169a20d71..92ad8fb1c52 100644
--- a/setuptools/dist.py
+++ b/setuptools/dist.py
@@ -16,6 +16,7 @@
 from distutils.debug import DEBUG
 from distutils.fancy_getopt import translate_longopt
 import itertools
+import textwrap
 
 from collections import defaultdict
 from email import message_from_file
@@ -49,11 +50,7 @@ def get_metadata_version(self):
     mv = getattr(self, 'metadata_version', None)
 
     if mv is None:
-        if (
-            self.long_description_content_type
-            or self.provides_extras
-            or self.long_description
-        ):
+        if self.long_description_content_type or self.provides_extras:
             mv = StrictVersion('2.1')
         elif (self.maintainer is not None or
               self.maintainer_email is not None or
@@ -71,7 +68,7 @@ def get_metadata_version(self):
     return mv
 
 
-def read_pkg_file(self, file):
+def read_pkg_file(self, file):  # noqa: C901
     """Reads the metadata values from a file object."""
     msg = message_from_file(file)
 
@@ -87,6 +84,24 @@ def _read_list(name):
             return None
         return values
 
+    def _read_long_description():
+        value = msg['description']
+        if value in ('UNKNOWN', None):
+            return None
+        description_lines = value.splitlines()
+        if len(description_lines) == 1:
+            return description_lines[0].lstrip()
+        description_dedent = '\n'.join(
+            (description_lines[0].lstrip(),
+             textwrap.dedent('\n'.join(description_lines[1:]))))
+        return description_dedent
+
+    def _read_payload():
+        value = msg.get_payload().strip()
+        if value == 'UNKNOWN':
+            return None
+        return value
+
     self.metadata_version = StrictVersion(msg['metadata-version'])
     self.name = _read_field('name')
     self.version = _read_field('version')
@@ -104,8 +119,9 @@ def _read_list(name):
     else:
         self.download_url = None
 
-    self.long_description = _read_field('description')
-    self.description = _read_field('summary')
+    self.long_description = _read_long_description()
+    if self.long_description is None and self.metadata_version >= StrictVersion('2.1'):
+        self.long_description = _read_payload()
 
     if 'keywords' in msg:
         self.keywords = _read_field('keywords').split(',')
diff --git a/setuptools/tests/test_dist.py b/setuptools/tests/test_dist.py
index e4bba47bce8..c948ea1bcaa 100644
--- a/setuptools/tests/test_dist.py
+++ b/setuptools/tests/test_dist.py
@@ -85,6 +85,9 @@ def __read_test_cases():
         ('Metadata version 1.1: Provides', params(
             provides=['package'],
         )),
+        ('Metadata Version 1.0: Short long description', params(
+            long_description='Short long description',
+        )),
         ('Metadata version 1.1: Obsoletes', params(
             obsoletes=['foo'],
         )),
@@ -162,6 +165,7 @@ def test_read_metadata(name, attrs):
         ('metadata_version', dist_class.get_metadata_version),
         ('provides', dist_class.get_provides),
         ('description', dist_class.get_description),
+        ('long_description', dist_class.get_long_description),
         ('download_url', dist_class.get_download_url),
         ('keywords', dist_class.get_keywords),
         ('platforms', dist_class.get_platforms),
diff --git a/setuptools/tests/test_egg_info.py b/setuptools/tests/test_egg_info.py
index 518b8392417..4df63ba6a03 100644
--- a/setuptools/tests/test_egg_info.py
+++ b/setuptools/tests/test_egg_info.py
@@ -855,9 +855,15 @@ def test_long_description_content_type(self, tmpdir_cwd, env):
         assert expected_line in pkg_info_lines
         assert 'Metadata-Version: 2.1' in pkg_info_lines
 
-    def test_description(self, tmpdir_cwd, env):
+    def test_long_description(self, tmpdir_cwd, env):
+        # Test that specifying `long_description` and `long_description_content_type`
+        # keyword args to the `setup` function results in writing
+        # the description in the message payload of the `PKG-INFO` file
+        # in the `<distribution>.egg-info` directory.
         self._setup_script_with_requires(
-            "long_description='This is a long description\\nover multiple lines',")
+            "long_description='This is a long description\\nover multiple lines',"
+            "long_description_content_type='text/markdown',"
+        )
         environ = os.environ.copy().update(
             HOME=env.paths['home'],
         )