Ten patients had spontaneous ruptures of the flexor digitorum profundus tendon involving the small finger. All ruptures occurred at work with the hand engaged in resisted flexion. Seven patients were treated by surgical means, four patients by direct repair, two by tendon transfer, and one by tendon graft. The ruptures were intratendinous occurring just distal to the A2 pulley in two patients, in the palm in four, and in the carpal canal in one. A pathologic condition to explain the tendon ruptures was not found on gross inspection or subsequent histologic examination. The seven patients treated by surgical means achieved satisfactory results. Follow-up averaged 52 months. Three patients did not have surgical repair and they have been followed-up for an average of 16 months. The results of direct repair within 1 week of rupture were superior to the results of late reconstruction.