From previous experimental investigations of a model of Legg-Calvé-Perthes disease in young pigs, it was speculated that the early radiographic phenomenon of the subchondral crescentic-shaped radiolucent line is a pathological fracture and, furthermore, that this fracture initiates the stage of resorption. Being painful, the fracture also heralds the clinical onset of the disease. From these laboratory investigations, plus a preliminary radiographic investigation in children, it has been observed that only that portion of the epiphysis underlying the subchondral fracture is resorbed. The radiographic investigation of 1,057 children (1,264 involved hips) with Legg-Calvé-Perthes disease from four institutions included 376 hips in which the diagnosis had been made early and the subchondral fracture was detectable. In all of these hips the extent of the subchondral fracture correlated precisely with the subsequent extent of maximum resorption. Consequently, in the early stage of Legg-Calvé-Perthes disease the extent of the subchondral fracture is of prognostic significance in predicting the eventual extent of involvement of the femoral head. We propose a simple and practical two-group classification of the extent of involvement of the femoral head: Group A (less than half of the head) and Group B (more than half of the head). The classification can be applied in the early stage of the disease when the subchondral fracture is detectable as well as throughout the ensuing resorptive stage.