One hundred and forty-six adults who had an isolated injury of the fibular collateral ligaments of the ankle were randomized to be managed operatively or non-operatively. Disruption of the ligaments was diagnosed by means of a physical examination and on the basis of stress radiographs of the ankle made with use of a specially designed device to hold the leg. Operative treatment, performed in seventy-three patients, consisted of suture repair of the disrupted ligaments within seventy-two hours after the injury, followed by immobilization of the ankle in a below-the-knee plaster cast for six weeks. Non-operative treatment, used for seventy-three patients, consisted of the use of an ankle orthosis for six weeks. After a minimum of two years of follow-up, we could detect no significant differences, with the numbers available, between the two groups with regard to the functional result or the degree of joint laxity that was evident on stress radiographs. The non-operative group lost a mean of 1.6 weeks from work, and the operative group lost a mean of 7.0 weeks. We concluded that non-operative treatment of an injury of the fibular collateral ligaments of the ankle yields results that are comparable with those of operative repair and is associated with a shorter period of recovery.