Abstract: A modular building system having two or more elongated hollow profiled frame members and female and/or male fasteners which separably connect the frame members to each other. Each frame member has smooth, plane and at least substantially uninterrupted external surfaces, at least one longitudinally extending stiffening core with a longitudinally extending at least partially tapped hole, at least one longitudinally extending outer wall adjacent an external surface and spaced apart from the core, and at least one pair of longitudinally extending sidewalls which connect the core with the outer wall. The outer wall, the core and the sidewalls define a longitudinally extending chamber which is located between the outer wall and the core and is flanked by the sidewalls. Male fasteners can extend transversely of the outer wall and core and into the holes in the cores of adjacent frame members or into transversely extending tapped bores of adjacent frame members.