Abstract: A latch-pin lock for sliding doors and windows has a latch (1) on a base plate (4) that is attachable to a sliding-door frame (5) adjacent to a sliding-door lock pin (7). The latch retains the sliding-door lock pin (7) in door lock-pin orifices (9) in opposition to vibrational manipulation of the door and door frame (5) generally applied by persons attempting unauthorized entry. The latch can be glued or otherwise fastened to the door frame (5) either after, before or during construction of a building to which the sliding door is attached. With either a pivotal or a sliding contact of the latch (1) with the base plate (4), the latch can be positioned on the lock pin (7) to retain the lock pin (7) in a locking or in an unlocking distance of entry into a lock-pin orifice (9) as desired.