Bilateral arthrography of the hip was performed in 37 patients with unilateral Legg-Calve-Perthes disease. Measurements made on the plain roentgenograms and on the arthrograms of the diameter of the femoral head, the width of the medial joint space, and the thickness of the acetabular cartilage were compared. The acetabulum-head quotients also were calculated. When present, evidence of flattening of the femoral head cartilage and of pooling of the contrast material also was noted. These data showed that in the early states of Legg-Calve-Perthes disease the articular cartilage is thickened, and that the cartilaginous femoral head is deformed and protrudes from the acetabulum. Arthrograms made with the hip in different positions also indicated the extent and location of collapse of the (osteochondral) femoral head and therefore proved useful as a means to determine the best therapeutic position of the hip to provide coverage of the head. In three patients with severe transient synovitis, thickening of the articular cartilage resembling that seen in Legg-Calve-Perthes disease was demonstrated.