A case of spontaneous rupture of the flexor hallucis longus tendon within the hind part of the foot is reported in a middle-aged woman who had no trauma or systemic disease. Repair was effected by tenodesis to the flexor digitorum longus tendon above and below the fibro-osseous tunnel. Hyperextension of the interphalangeal joint which most troubled the patient was corrected postoperatively. Active range of motion was equal in extension. Flexion at the IP joint was present, but significantly less than the unaffected side.