diff --git a/include/boost/mpl/aux_/config/use_preprocessed.hpp b/include/boost/mpl/aux_/config/use_preprocessed.hpp
index 2ed83dd..0165449 100644
--- a/include/boost/mpl/aux_/config/use_preprocessed.hpp
+++ b/include/boost/mpl/aux_/config/use_preprocessed.hpp
@@ -17,6 +17,6 @@
 #ifndef BOOST_MPL_AUX_CONFIG_USE_PREPROCESSED_HPP_INCLUDED
 #define BOOST_MPL_AUX_CONFIG_USE_PREPROCESSED_HPP_INCLUDED
 
-//#define BOOST_MPL_USE_PREPROCESSED_HEADERS
+#define BOOST_MPL_USE_PREPROCESSED_HEADERS
 
 #endif // BOOST_MPL_AUX_CONFIG_USE_PREPROCESSED_HPP_INCLUDED