As indicated by http://www.boost.org/development/tests/trunk/developer/output/cm_x64-boost-b.... In boost/atomic/detail/interlocked.hpp, BOOST_ATOMIC_LLONG_LOCK_FREE must be defined to 2 if _WIN64 is defined (cas64strong.hpp is included afterwards). Because the library has not yet been released, I didn't bother to create a ticket and because the change is so trivial I didn't create a patch as well (if either or both is required or wanted, please let me know). Question: If I recall correctly, long long can be made lockfree also in 32-bit. If I am right, is there a reason why it hasn't been done? Thanks. --Christoph.