A programmable cylinder lock includes a stator and a cylindrical rotor, mounted therein for rotation around its own axis, has a keyhole, and comprising inside the rotor key followers movable along the longitudinal and transversal directions, and locking pins movable along the longitudinal direction, the followers and locking pins forming together a number of pairs and having toothings for mutually cooperating to define the lock codification, the rotor including a stop bar cooperating with a longitudinal groove of the stator and cooperating with the pins to immobilize them, and a change bar slidingly coupled with the followers to keep them engaged with the pins and to disengage them therefrom when the bar provides a lock programming position. The followers have a projection which engages a side surface of the key and, at the contact of a normal use key, prevents the transversal displacement of the key and the change bar.