The radiographic changes in 57 patients with Legg-Perthes disease were studied to define the patterns of progression of one disease stage to the next. Femoral neck width, epiphyseal width and height, acetabular depth, “tear-drop” distance, and femoral head width and sphericity in the anteroposterior projection were measured on 529 X-ray films. Two distinct groups of hips were found. Group I (43) hips showed moderate changes in all parameters during fragmentation phase but improved in subsequent years, maintaining a concentric reduction and adequate sphericity in the femoral head. Group II (14) hips were initially identical to the hips in Group I. During mid-fragmentation phase, however, “tear-drop” distance increased suddenly and significant flattening and widening of the epiphysis rapidly followed. There was loss of the normal congruent relationship of acetabulum and femoral head with progressive increase in femoral head size and a marked loss of sphericity of the femoral head. Surgical reestablishment of an adequate femoral-acetabular relationship reversed the tendency toward further flattening in many cases.