A pathologic rupture of the flexor pollicis longus tendon secondary to Kienbock disease is extremely rare with only three cases being reported in the literature. We encountered a case of a pathological rupture of the flexor pollicis longus tendon secondary to longstanding Kienbock disease. The treatment included tendon ball insertion after excising the collapsed lunate and a flexor pollicis longus tendon reconstruction with autogenous palmaris longus tendons. Satisfactory results with a restoration of the active motion of the interphalangeal joint of the thumb, maintenance of the preoperative active range of motion of the wrist and markedly reduced pain was achieved after a one year follow up. We describe this case with a review of the relevant literature.