A 2-dimensional motion and force study of the ankle joint during gait has been carried out on normal subjects and patients with ankle joint disease, before and 1 year following total ankle replacemetn. The methods employed involved the use of high-speed motion picture film, force plate and foot-switch data. The Achilles and anterior tibial tendon forces, the compressive and tangential (shear) forces across the ankle during stance phase of gait were determined, based on a quasi-static analysis. During stance phase of gait normal subjects used a mean of 24.4 degree of sagittal plane ankle motion. Patients with ankle joint disease showed reduced motion which returned to near normal values 1 year following total ankle replacement. Compressive force across the ankle joint rose to about 5 times body weight during the latter part of stance phase. Backward, or aft, shear forces or nearly full body weight were demonstrated during all but the last 20% of stance phase. Patients with ankle joint disease apparently altered their gait to markedly reduce these forces. Following total ankle replacement, shear forces returned toward more normal values, but compressive forces were not significantly changed.