Recent advances in the understnading of the biomechanics of the ankle have given rise to the clinical uncertainty about the indications for the operative treament of isolated fractures of the lateral malleolus. If deltoid ligament injury is associated, it may be unstable and operation may be indicated. This study was done to determine if we are able to assess the stability of the isolated lateral malleolus fracture based on the fracture patterns seen on radiographs and clinical findings. 37 patients with malleolar fracture of the ankle were treated at Hanil General Hospital by open reduction and internal fixation from Dec. 1996 to Jan. 1998. Lauge-Hansen classification was tried in all cases to determine if it could be applied. Stress test under anesthesia on 10 isolated lateral malleolar fracture patients with clinical findings of injury on the deltoid ligament area, whose medial clear space were normal or widened less than 2 mm on initial film. Exploration of the deltoid ligament was performed in 6 of above 10 patients There were 21 supination-external rotation type injuries, 2 supination-adduction injuries, 1 pronation-abduction injury and 13 fractures could not be clearly categorized into specific group. In 6 among 13 unclear cases, there were short oblique fracture lime indicating pronation injury, but direction of the fracture line could not be clearly determined. In 4 cases, direction of the fracture lime indicated pronation injury, but the length of the fracture line was too long. In 3 cases, both the direction and length of the fracture line was indeterminate. The stress test on 10 isolated lateral malleolar fraactures with clinical findings of deltoid injury revealed less than 2 mm widening in 7 cases, more than 2 mm widening in 3 cases. In 3 cases with less than 2 mm widening, tearing of anterior third of superficial deltoid was confirmed by exploration. In 3 cases with more than 3 mm widening, partial rupture of the deep deltoid was observed in 2 cases and anterior third of superficial deltoid was torn in 1 case. We suggest that we cannot clearly assess the stability of isolated lateral malleolar fracture either by fracture patterns or clinical findings, so stress test can be considered in determining the stability.