A 20-year-old male athlete had painful triggering and occasional locking of the great toe caused by entrapment of the flexor hallucis longus tendon within the flexor tendon sheath posterior to the left medial malleolus. No rupture of the tendon fibers, tendonitis, or supernumerary muscle was found. At open operation an elongated flexor hallucis longus was found entrapped under the tendon sheath in the ankle. After other therapy methods failed, the condition was relieved by a surgical procedure that released the flexor hallucis longus muscle from the tendon sheath.