The treatment of acquired flat foot secondary to dysfunction of the posterior tibial tendon (PTT) of stage II, as classified by Johnson and Strom, remains controversial. Joint sparing and soft-tissue reconstructive procedures give good early results, but few studies describe those in the medium-term. We studied prospectively the outcome of surgery in 51 patients with classical stage-II dysfunction of the PTT treated by a medial displacement calcaneal osteotomy and transfer of the tendon of flexor digitorum longus. We reviewed 44 patients with a mean follow-up of 51 months (38 to 62). The mean American Orthopaedic Foot and Ankle Society ankle/hindfoot rating scale improved from 48.8 before operation to 88.5 at follow-up. The operation failed in two patients who later had a calcaneocuboid fusion. The outcome in 43 patients was rated as good to excellent for pain and function, and in 36 good to excellent for alignment. There were no poor results.