Thirty-four patients were studied throughout the evolution of Perthes' disease. The acetabular changes included osteopoenia of the roof, irregularity of its contour, and decrease in its depth. These changes were proportional to the femoral head involvement. The purpose of the study was to assess the effect of the morphological changes of the femoral head on the acetabulum and the outcome, and to determine the extent to which coxa magna and acetabular enlargement induced by Perthes disease in childhood persist into adolescence. Radioisotope scans of the hip were examined in fourteen children with unilateral Perthes' disease and comparison was made with the contralateral hip. These scans showed increased uptake on the lateral part of the acetabulum and no uptake over the avascular part of the femoral head. Average follow-up was ten years and children were followed up on average from six years to fifteen years of age. Six readings of the measurements of various dimensions of the acetabulum and the femoral head were done. CT scan also showed irregularity in the acetabulum. Statistical tests lead to the conclusion that the decrease in the depth of the acetabulum was secondary to the femoral head involvement and the extent of its dimensional changes affected the final congruity between the femoral head and the acetabulum. Also the remodelling potential of the acetabulum decreases as the child grows older. Therefore containment procedures could be done by femoral osteotomy in younger children, whereas acetabular osteotomy may benefit older children.