The flexor hallucis longus (FHL) tendon is susceptible to injury along its entire course from the posterior aspect of the ankle to its insertion into the base of the distal phalanx of the great toe. Various lacerations, ruptures, longitudinal splits, and stenosing tenosynovitis have been noted. This report documents three cases of longitudinal split of the FHL at the knot of Henry. The diagnosis of this entity is based solely on history and physical examination. Patients with this problem have experienced either an acute or chronic repetitive hyperextension of the hallux metatarsophalangeal joint. They complain of pain with prolonged walking and running and have tenderness with palpation of the knot of Henry (the anatomical crossover between the FHL and the flexor digitorum longus) about one thumb-breadth lateral to the tuberosity of the navicular. Noninvasive imaging studies, including ultrasound and magnetic resonance imaging, are not helpful in establishing this diagnosis. Surgical treatment includes release of the knot of Henry, debridement and repair of the longitudinal split in the FHL, and excision of the interconnecting tendon between the FHL and the flexor digitorum longus. All three patients presented in this report have obtained long-term satisfactory relief of their symptoms with surgical treatment.