PURPOSE: To evaluate the results of conservative treatment for isolated lateral malleolus fracture without medial ankle injury. MATERIALS AND METHODS: From March 1999 to February 2003, 25 ankles in 25 patients were treated for isolated lateral malleolus fracture and followed for more than one year. Mean age was 46.9 years (range, 20~71 years). Cases without any swelling or tenderness on the deltoid area, or cases with minimal pain, swelling or tenderness on the deltoid area and medial clear space 1 mm or less on stress radiograph were included for the study. Immediate weight bearing was allowed with below-knee cast immobilization in all cases. RESULTS: All were supinatin-external rotation stage II injury and mean duration of cast immobilization was 6.3±1.6 weeks after injury. There was no case which showed widening of medial clear space during routine radiographic follow-up. There was no change in the degree of displacement in spite of immediate weight bearing with short leg cast on. CONCLUSION: Because the lateral malleolus fracture without medial injury can be managed nonoperatively, we need to differentiate this type of fracture to avoid unnecessary surgery, and for early return to normal daily activity.