Abstract: A safe and tamper-resistant lock assembly for sliding glass doors and the like for securely locking a slidable door in a closed or partially opened position. The lock assembly includes a specially configured bracket which is mounted upon the stationary frame assembly of the door unit. The bracket has an inverted J-shaped channel and an elongated wall defining a plurality of apertures including one aperture which is positioned to overlie an aperture in the slidable frame of the door when the door is in a closed position and another aperture which is positioned to overlie the aperture in the slidable frame when the door is in a partially opened position. A specially configured locking key is provided with a cylindrical shank having an arcuate locking cam for lockably engaging the inverted J-shaped channel of the bracket when the shank is inserted into the aperture in the slidable frame and into one of the apertures in the bracket.