Causes of failure of 85 allografts in a series of 123 cadaver kidney transplantations were analyzed. Fifty-four failures (62%) occurred during the first three postoperative months, of which more than half occurred during the first month. Rejection (43.5%) was the most common cause of failure of an allograft, whereas infection was the most frequent cause of death of the host. Thrombosis of the renal artery occurred in eight of ten allografts when ABO blood groups were crossed from non-O-type donor to O-type recipient, but this complication was not seen when blood groups were crossed from O-type donor to non-O-type recipient. All recipients well matched by leukocyte antigens, and more than half of those who were mismatched, had good graft function and minimal rejection episodes. Thymectomy and splenectomy made no difference in the incidence of rejection, but predisposed the patients to fatal infections.