The results of clinical renal transplantation are determined mainly by immunological factors, the most important of which are compatibility for the HLA chromosomes and pretransplant blood transfusions. Other factors include HLA matching in cadaver transplantation, compatibility for the Lewis blood group system, and sensitization to lymphocyte panels or to endothelial-monocyte antigens. Performance of the previous graft is the most reliable predictor of success in recipients of retransplants. Because several immunological factors may interact, multiple variables must be considered simultaneously.