diff --git a/recommonmark/__init__.py b/recommonmark/__init__.py
index abeeedb..9e4ac21 100644
--- a/recommonmark/__init__.py
+++ b/recommonmark/__init__.py
@@ -1 +1,8 @@
 __version__ = '0.4.0'
+
+
+def setup(app):
+    """Initialize Sphinx extension."""
+    from .parser import CommonMarkParser
+    app.add_source_parser('.md', CommonMarkParser)  # needs Sphinx >= 1.4
+    return {'version': __version__, 'parallel_read_safe': True}