As a newbie I'd like to say a big thank to all those concerned for boost. I found that the following (MSVC6) program fails when in the (default) debug build, but works in release: #include <iostream> #include int main(int , char* ) { int NUM_CLIENTS = 3; boost::thread_group thrds; std::cout << NUM_CLIENTS; // prints 0 in _DEBUG build, 3 in release return 0; } It was linked using the libboost_thread.lib file created using the following bjam command: bjam "-sTOOLS=msvc-stlport" "-sSTLPORT_PATH=F:\TemplateStuff" "-sBUILD=debug <runtime-link>static <stlport-iostream>on" POSSIBLE FIX Compiling the above program with _STLP_DEBUG defined gives the correct answer. Is this the correct fix to the problem? (I've noted that there is no mention of _STLP_DEBUG in the distributed docs) Mike Wilson Thanks again to the boost community.