diff --git a/include/boost/thread/thread.hpp b/include/boost/thread/thread.hpp
index f6ad5856..a3575ee2 100644
--- a/include/boost/thread/thread.hpp
+++ b/include/boost/thread/thread.hpp
@@ -65,7 +65,7 @@ public:
     void add_thread(thread* thrd);
     void remove_thread(thread* thrd);
     void join_all();
-        int size();
+        int size() const;
 
 private:
     std::list<thread*> m_threads;
diff --git a/src/thread.cpp b/src/thread.cpp
index 557f7d92..185f45f5 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -378,7 +378,7 @@ void thread_group::join_all()
     }
 }
 
-int thread_group::size()
+int thread_group::size() const
 {
         return m_threads.size();
 }