The clinician should be aware of the possibility of a rupture of the tibialis posterior tendon in adults who have developed a unilateral flatfoot deformity. By careful physical examination, the diagnosis can readily be made. In more advanced cases roentgenographic changes of medial subluxation of the talar head and evidence of sagging of the talonavicular joint are noted on the lateral view. If conservative treatment fails to result in a satisfactory clinical response, I believe the treatment of choice is a flexor digitorum longus transfer through the navicular. If this procedure fails, bony stabilization of the hindfoot may be indicated.