This experimental investigation was performed to study the development of a subchondral fracture after ischemic osteonecrosis of the immature femoral head using a piglet model. Forty-eight male piglets were studied after placing a ligature tightly around the femoral neck to disrupt the blood supply to the femoral head. Animals were euthanized 2–8 weeks after the induction of ischemia. Radiographic, histologic, and histomorphometric assessments were made. A subchondral fracture was seen in 12 out of 32 infarcted femoral heads (38%) that were in the initial radiographic stage of ischemic osteonecrosis. The fracture was seen mainly in those femoral heads that had a prolonged period of the initial stage where the initiation of revascularization and repair was delayed. Histomorphometric assessment showed decreased trabecular thickness and volume in the subchondral region of the infarcted femoral heads compared with the contralateral normal heads. After ischemic osteonecrosis, the trabecular bone in the subchondral region is thinner and less bone volume is present because of a lack of new bone formation. The results of this study support the hypothesis that a subchondral fracture in the immature femoral head develops as a result of mechanic failure of the trabecular bone in the subchondral region.