Flexor hallucis longus (FHL) tendon transfer to the calcaneus in the repair of delayed or neglected Achilles tendon (AT) injuries is a viable and dynamic option. Nineteen patients (18 males, 1 female; mean age 47.4 +/- 12.4, range 24 to 74, years; body mass index 27.5 +/- 4.5, range 23.2 to 38.9, kg/m(2): interval from injury to surgery 40.8 +/- 11.6, range 28 to 60, days) with delayed or neglected repair of AT rupture were included in the present study. FHL transfer to the calcaneus through a single incision and repair of the defect with native tendon lengthening or a tendinous turndown flap was performed. American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot and hallux scale scores, balance and jump performance, ankle dorsiflexion range of motion, and lower extremity concentric and eccentric strength were evaluated 6 months postoperatively. Student's t test was used to compare the outcomes between the operated and nonoperated sides. AOFAS hindfoot and hallux scale scores were 93.83 and 86.9, respectively. No significant difference was found in vertical jump (p = .60), forward jump (p = .68), or balance performance (p> .05). However, less ankle dorsiflexion on the operated side was recorded compared with the nonoperated side (p = .008). Concentric/eccentric muscle strength between the operated and nonoperated side was similar (p > .05). The concentric strength of the operated side reached 92% and eccentric strength reached 101.7% of the nonoperated side's strength. All the patients were satisfied with their results and return to preinjury daily activities. AT repair of a delayed and neglected injury using FHL transfer to the calcaneus in a dynamic fashion provided excellent outcomes. (C) 2018 by the American College of Foot and Ankle Surgeons. All rights reserved.