BACKGROUND: In patients with chronic Achilles tendinopathy, augmentation with flexor hallucis longus (FHL) tendon transfer can be performed to improve pain and functional limitations. There are no reports of postoperative imaging for evaluating tendon integration, inflammatory alterations or degeneration of the FHL muscle. The purpose of this study was to evaluate postoperative MR imaging based on clinical outcome and isokinetic strength. MATERIALS AND METHODS: 13 patients with chronic Achilles tendinopathy (10 ruptures) underwent augmentation with FHL transfer. Clinical parameters, isokinetic strength and outcome measurements (AOFAS, SF-36) were evaluated at an average followup of 46.5 months. Qualitative and quantitative analyses of postoperative MRI were conducted using the non-operated side for comparison. RESULTS: All patients had a significant reduction of pain. The operated side had a torque deficit of 35% for plantar flexion. Ten patients returned to their former level of activity. MRI showed a complete integration of the FHL tendon in six patients. Fatty atrophy in the triceps surae was found in ten patients. The FHL was free of degeneration in all patients. Hypertrophy of the FHL of more than 15% was observed in eight patients. CONCLUSION: Augmentation with FHL transfer is a valuable option in the treatment of chronic Achilles tendinopathy with and without rupture. Our results demonstrate high patient satisfaction without donor site morbidity. The FHL tendon is well integrated into the Achilles tendon. Hypertrophy of the FHL muscle suggests functional incorporation into plantar flexion. The primary benefit of the operation is pain relief and increased muscle strength.