Background: While an os trigonum at the posterolateral aspect of the talus is usually asymptomatic, this inconsistently present accessory bone has been associated with persistent posterior ankle pain, which has been described as the os trigonum syndrome. We present the clinical results of excision of the os trigonum through a posterolateral approach and report several factors affecting the clinical outcome. Methods: During a five-year period from 1994 through 1999, forty-one patients had a failure of nonoperative treatment of os trigonum syndrome and underwent excision of a symptomatic os trigonum. In all cases, the os trigonum syndrome was diagnosed on the basis of the history, physical examination, and radiographs. Postoperatively, the patients were evaluated according to the American Orthopaedic Foot and Ankle Society (AOFAS) Ankle-Hindfoot Scale. A questionnaire was used to evaluate the effect of several factors on the clinical outcome. Results: The average duration of follow-up was forty-four months. The postoperative AOFAS score averaged 87.6 points. The thirty-three patients who had had symptoms for two years or less prior to the surgery had an average postoperative score of 90 points compared with 78 points for the eight patients who had had preoperative symptoms for more than two years (p = 0.011). Eight patients had sural nerve sensory loss, which was temporary in four and permanent in four. A superficial wound infection developed in one patient, and reflex sympathetic dystrophy developed in another. Conclusions: An os trigonum that is persistently symptomatic after a minimum three-month trial of nonoperative treatment can be excised through a posterolateral approach with highly satisfactory results. The main complication of this procedure is sural nerve injury. Level of Evidence: Therapeutic study, Level IV (case series [no, or historical, control group]). See Instructions to Authors for a complete description of levels of evidence.