Two groups of patients who had Legg-Calvé-Perthes disease were studied. The first group of patients consisted of eighty-eight patients (ninety-nine affected hips) followed in three hospitals for an average of forty years. The second group consisted of sixty-eight patients (seventy-two affected hips), all of whose radiographs from the onset of disease to maturity were available and all of whom had been treated in one hospital. The patients in this second group were followed for an average of thirty years. Each hip in both study groups could be placed into one of five classes of deformity based on its radiographic appearance at maturity. Each class showed a characteristic pattern of involvement during the active stages of the disease and had a specific long-term clinical and radiographic course. The clinical and radiographic course of an involved hip subsequent to childhood was related to the type of congruency that existed between the femoral head and acetabulum. Three types of congruency were recognized: (1) spherical congruency (Class-I and II hips) - in hips in this category arthritis does not develop; (2) aspherical congruency (Class-III and IV hips) - mild to moderate arthritis develops in late adulthood in these hips; and (3) aspherical incongruency (Class-V hips) - severe arthritis develops before the age of fifty years in these hips.