The writers describe one case of idiopathic chondrolysis of the hip and four cases of chondrolysis secondary to slipped upper femoral epiphysis, all followed up from two to six years. So far as the writers know, idiopathic chondrolysis has not previously been reported in the literature in a male of the white race after the end of skeletal growth. In this case biopsy disclosed necrosis and fibrosis of the articular cartilage with extensive infiltration of lymphocytes and plasma cells in the synovial membrane and in the subchondral medullary spaces. In all the cases described there was gradual regression of pain and improvement in the radiographic appearances within about one year. Moderate limitation of movement persisted, however, especially internal and external rotation. In all cases the treatment consisted of protected weight bearing and physiotherapy.