Findings from biomechanical analyses of gait were used to estimate the optimum position of arthrodesis of the ankle. Nineteen patients who were followed for an average of 10.4 years (range, four to seventeen years) were studied. By including the knee in the analysis as well as studying the effects on gait of different ground conditions, objective data for the weight-bearing extremities in the transverse, sagittal, and coronal planes were generated. Genu recurvatum was shown to be associated with a plantar-flexion position of fusion of the ankle. Laxity of the medial collateral ligament of the knee was noted in twelve patients (63 per cent). Among these patients, in three (16 per cent) the laxity was graded as moderate to severe, possibly due to external rotation of the extremity during gait to avoid rolling over the rigid plantar-flexed ankle. The patterns of gait showed that a valgus position of the arthrodesis is more advantageous and provides more normal gait, particularly on uneven ground. To attain more normal function of the knee and improve performance on rough ground, the optimum position of arthrodesis of the ankle appears to be neutral flexion, slight (zero to 5 degrees) valgus angulation, and approximately 5 to 10 degrees of external rotation. Posterior displacement of the talus under the tibia tends to produce a more normal pattern of gait and decreases the stress at the knee. CLINICAL RELEVANCE: This study has shown the ideal position of fusion of the ankle to be neutral flexion, slight (zero to 5 degrees) valgus angulation of the hind part of the foot, and 5 to 10 degrees of external rotation. This position allows the greatest compensatory motion at the foot and places the least strain on the knee.