Abstract: A security door for use with a sliding glass door assembly that is installed in a casing, comprises a frame adapted for rotatable connection to the casing about a generally vertical axis, and first and second panels connected to the frame. The frame includes an upper guide track and a lower guide track that extend in a generally horizontal direction. The first panel is fixedly connected to the upper and lower guide tracks, while the second panel is slidably connected to the upper and lower guide tracks for sliding movement with respect to the first panel between a retracted position wherein the first and second panels are in opposing relationship, and an extended position wherein the trailing edge of the second panel is proximal the leading edge of the first panel. The upper guide track has a length that is approximately equal to a distance between the trailing edge of the first panel and the leading edge of the second panel with the second panel in the extended position.