Osteonecrosis of the distal femur occurs more often in the medial femoral condyle than in the lateral femoral condyle, but the vascular supply to these regions has not previously been described. Twelve fresh adult cadaver legs were injected with india ink or latex via the femoral artery and then meticulously dissected to evaluate the extraosseous blood supply. After all soft tissue was removed, the intraosseous blood supply was evaluated using a modified Spateholtz technique. The vascular structures at risk during posterior cruciate ligament reconstruction were also identified. The analysis of the extraosseous arterial supply demonstrated that the superior and inferior lateral genicular arteries combine to supply the lateral femoral condyle. The medial femoral condyle is supplied primarily by the superior medial genicular artery and other lesser branches of the popliteal artery. The intraosseous supply to the lateral condyle was shown to consist of an arcade of vessels providing multiple branches to the subchondral bone with no obvious "watershed" region of limited vascularity. The intraosseous supply to the medial condyle appeared to consist of a single nutrient vessel supplying the subchondral bone with an apparent watershed area of limited supply. A potentially significant difference exists between the intraosseous and extraosseous blood supply to the medial and lateral femoral condyles that may explain the higher frequency of ischemic events occurring in the medial femoral condyle. The close proximity of the extraosseous vessels to the medial femoral condyle and the standard femoral tunnel used during posterior cruciate ligament reconstruction may explain the occurrence of avascular necrosis after this procedure.