The limping gait due to the leg length discrepancy or/and coxa vara in Legg-Perthes' disease is caused by the disturbance on the proximal growth plate of femur. However, there have been insufficient reports of growth disturbace of proximal femur. The authors have analysed 79 unilateral Legg-Perthes' disease treated by femoral varus derotation osteotomy and report on 37 patients who had at least 3 years follow-up, with focus on the articulotrochanteric distance index (ATDI) on both hip anteroposterior radiograph. The results were as follows;1. The materials consisted of 33 boys and 4 girls with 4 cases which belong to group II, 23 cases to group III and 10 cases to group IV accordings to Catterall classification. 2. The comparison of the ATDI among Catterall group were that all 4 cases of group II were more than 0.75. In group II, 12 cases were more than 0.75, 6 cases between 0.5 to 0.75 and 5 cases below 0.5. In group IV, 3 cases more than 0.75, 4 cases between 0.5 to 0.75 and 5 cases below 0.5. 3. The more extensive involvement of femoral head, the more coxa magna and coxa plana were found. ATDI was directly proportional to the damage of growth plate of proximal femur. 4. Of the eight cases with ATDI less than 0.5, limping gait was shown in 6 cases, abductor weakness in 5 cases, whereas limping gait and abductor weakness were in 3 cases of each other of 10 cases with ATDI between 0.5 to 0.75. The measurement of ATDI is believed to be a significant index to predict the abductor weakness, the degree of head involvement, deformity.