Biopsy specimens of the femoral head and neck of two patients with Legg-Perthes disease have shown a necrotic femoral head and a disrupted epiphyseal plate. The epiphyseal-plate cartilage was fibrillated and cracked, the cells were clustered, and the line of endochondral ossification was irregular. The epiphyseal-plate lesion appeared to be a long-standing condition, as large tongues of deranged cartilage and cartilage islands were seen very deep in the metaphysis and some in the femoral head. There was some fibrillation in areas of the joint cartilage adjacent to the subchondral plate. There were no signs of infection, rickets, or scurvy. The epiphyseal-plate lesion appears to be caused by a loss of cohesion of the cartilage matrix presumably owing to an alteration of the chemical composition of the ground substance. It is surmised that the loss of blood supply to the femoral head results from the interruption of the retinacular vessels when they cross the cartilage at the edge of the disrupted epiphyseal plate. Jarring of the weakened plate may traumatize and occlude these vessels. The epiphyseal-plate lesion in these two patients is similar to the lesion observed in patients with slipping of the upper femoral epiphysis. It is also similar to the epiphyseal-plate lesion observed in rats fed diets containing aminonitriles.