In a study of 1360 cadaver-donor kidney transplants we found a striking correlation of increased numbers of pretransplant blood transfusions with improved transplant survival (P less than 0.0001). Graft survival rate in recipients with greater than 20 transfusions was 71 p5 per cent at one year as compared with 42p2 per cent for recipients with no transfusions; at four years the survival rates were 65p5 per cent and 30p3 per cent (P less than 10(-6). Frozen blood was less effective than nonfrozen blood in producing this effect. In contrast to previous reports based on fewer numbers of transplants, a single pretransplant transfusion of transfusions given during transplantation had no statistically significant influence on graft outcome. The beneficial effect of pretransplant transfusions was apparent at transplant centers with high or low overall success rates. Deliberate transfusion trials in prospective transplant recipients should consider this strong dose dependence of graft prolongation by transfusions.