Biopsies specimens from the femoral neck and head of three patients with early slipping of the upper femoral epiphysis were obtained. The epiphyseal plate in these patients was found to be wide and greatly disrupted, and clefts had occurred in areas of the plate. There were no signs of rickets, osteomalacia, osteoporosis, or infection. The epiphyseal-plate lesion appears to be due to loss of cohesion of the cartilage matrix, presumably caused by an alteration of the chemical composition of the ground substance. This lesion appears to be mainly responsible for the epiphyseal slipping. Metabolic studies of these children have shown an abnormality in the protein metabolism similar to the one found in children with adolescent scoliosis. The epiphyseal-plate lesions in these patients were similar to the lesions observed in experimental animals fed minimal amounts of aminonitriles.