Long-standing lateral ligament instability of the ankle results in unbalanced loading of the medial joint space and the possible development of degenerative arthritis. Thirty-six patients who had had lateral ankle instability for at least ten years and complaints of increasing ankle pain evidence degenerative changes of the articular cartilage over the medial half of the talar and tibial surfaces of the ankle joint. These changes were minimally apparent on roentgenograms unless weight-bearing roentgenograms were made, but arthroscopy of the joint accurately revealed the extent of degeneration. After reconstruction of the lateral ankle ligaments, fourteen of twenty-two patients with mild to moderate arthritic changes showed both symptomatic improvement and demonstrable widening of the medial joint space on weight-bearing roentgenograms. Four of five patients with severe degenerative arthritis subsequently had a total ankle replacement.