Abstract: A security screen assembly comprising a sub-frame mounted within an opening in a building and a hinged main frame which has a screened opening. Keys are provided at each corner of each frame to maintain the frame members in a predetermined angular relationship. The hinge is designed to permit assembly and disassembly of the hinged parts by a movement through an open side of one of the hinge parts. The screen is marginally held within the opening in the pivoted frame by screws extending through the margin of the screen which self-tap into a slot in the frame members, and ribs and slots on the main frame and sub-frame are provided to prevent warping of the parts in the closed position.