Abstract: A screw lock assembly is provided for sash windows. The lock comprises a self-tapping screw for insertion into aligned apertures formed in the abutting frame members of the window. The aligned apertures are open at one end adjacent the inside of the window and the inner aperture has a substantially enlarged portion at the open end. A cup washer is countersunk by positioning in the enlarged portion and the head and flat washer of the screw is recessed into the cup washer to limit accessibility to the head when the screw lock is in the operative position. Circular ridges are formed on the underside of the flat washer and serve as a locking means by frictionally engaging the adjacent face of the cup washer. The head and flat washer is integral with the screw and the head is made tamper-proof by the act that it is a cylinder with a single, substantially ninety degree flattened side. A key is provided having a corresponding outline for driving the screw.