A radiographic survey of 25 patients with bilateral Perthes disease is compared with four inherited skeletal dysplasias also affecting the hip joints (45 patients with multiple epiphyseal dysplasia, 22 with spondyloepiphyseal dysplasia tarda, 18 with pseudoachondroplasia, and 17 with spondyloepiphyseal dysplasia congenita). The distinguishing features in relation to the pelvis and hip joint in the growing child are ascertained, in view of the importance of differentiating as early as possible the transient disorder of Perthes disease from the more serious progressive disorders.