We evaluated the cases of twenty-six patients (twenty-six ankles) who had had revision of an ankle arthrodesis with external fixation for a nonunion, to determine the reasons for the failure of the previous arthrodesis. Eighteen patients had had supplemental bone-grafting in addition to the external fixation. The failure of the previous arthrodesis was related to inadequate fixation technique in seven patients and to technical problems in two patients; in the other seventeen patients at least one risk factor was identified. We also determined the functional results of the revision operation with external fixation for all patients. The average duration of follow-up was five years (range, two to ten years) in the twenty-two patients who did not have a reoperation for a persistent nonunion. The results were excellent in eleven patients, good in five, fair in four, and poor in six. The over-all rate of union was twenty (77 per cent) of twenty-six, comparable with that after primary arthrodesis; however, supplemental bone-grafting is usually necessary. In the current series, rigid fixation, precise apposition of bone and alignment of the foot, and early treatment of perioperative infection gave satisfactory results.