We investigated the revascularization pattern of patellar tendon grafts used to replace the anterior cruciate ligament in thirty-six dogs by histological and tissue-clearing (Spalteholz) techniques. Initially the grafts were avascular, but by six weeks they were completely ensheathed in a vascular synovial envelope. The soft tissues of the infrapatellar fat pad, the tibial remnant of the anterior cruciate ligament, and the posterior synovial tissues contributed to this synovial vasculature. Intrinsic revascularization of the patellar tendon graft progressed from the proximal and distal portions of the graft centrally and was complete by twenty weeks. The tibial attachment of the patellar tendon graft did not contribute any vessels to the revascularization process. At one year, the vascular and histological appearance of the patellar tendon graft resembled that of a normal anterior cruciate ligament. CONCLUSIONS The absence of perfused vessels within the patellar tendon graft immediately after transplantation within the knee joint and the failure of the osseous insertion of the graft to contribute vessels to the revascularization process suggest that although it is left attached at the tibia, the patellar tendon graft is essentially an avascular free graft at transplantation. The contribution of the soft tissues of the knee to the revascularization process of the graft suggests preservation and utilization of the infrapatellar fat pad and synovial tissue to optimize the graft's revascularization and ultimate viability.