Radiographs of 20 patients with acute chondrolysis complicating a slipped capital femoral epiphysis were reviewed with emphasis on the radiographic features and differential diagnosis. There was bilateral disease in five instances; radiographic follow-up ranged from 6 months to 5 years. The onset of chondrolysis occurred after surgery in 18 of the 25 abnormal hips. Acute necrosis of the articular cartilage is characterized radiographically by (1) persistent juxtaarticular osteoporosis; (2) progressive narrowing of all or a portion of the cartilage space; and (3) erosion of the subchondral cortices of the femoral head and acetabulum. The radiographic differential diagnosis varies with the stage of disease. The initial osteoporosis is indistinguishable from disuse. The rapid loss of the joint space, accompanied by destruction of the central portions of the subchondral bone, makes differentiation from postoperative infection a common problem. Late changes may resemble either degenerative joint disease or a chronic inflammatory arthritis. The accurate diagnosis of acute chondrolysis depends on correlation of the clinical history and radiographic findings.