The reason why late degenerative arthritis developed in some patients who had sustained displaced bimalleolar fractures of the ankle was investigated. The roentgenograms indicated that incomplete reduction of the lateral malleolus and a residual talar tilt were present. When bimalleolar fractures were created in cadavera the talus could be anatomically repositioned only when the lateral malleolus was accurately reduced. Fifty-three patients with bimalleolar fractures were treated by anatomically fixing the lateral malleolus with a four-hole plate. There was an anatomical reduction of the talus and medial malleolus in each instance and there were no late cases of degenerative arthritis when these patients were followed for from six months to nine years. We concluded that the lateral malleolus is the key to the anatomical reduction of bimalleolar fractures, because the displacement of the talus faithfully followed that of the lateral malleolus.