Thirty-six hip joints from cadavera ranging in age from seven fetal months to three years were studied. During the first year of life there is a diffuse, canalicular vascular network, primarily end-arterial, within the chondroepiphysis, with several arteriosinusoidal vessels crossing the growth plate. Initially the proximal femoral chondroepiphysis and growth plate are supplied approximately equally by lateral circumflex (anterior half) and medial circumflex (posterior half) arterial branches. Subsequent vascular development is characterized by regression of the lateral circumflex system and concomitant development of the medial circumflex system. A few large intra-articular vessels coursing along the posterior portion of the femoral neck enter the capital femoral epiphysis. The transition from lateral circumflex to medial circumflex arterial supply renders the anterior and lateral segments of the capital femoral epiphysis and growth plate more susceptible to vascular compromise, because the initial course of the medial circumflex artery is between the iliopsoas and adductor longus muscles, and then between the iliopsoas and pubic ramus, and positional extremes may partially or completely occlude the medial circumflex artery between these structures.