Delayed hyperacute rejection, with its characteristic clinical course and histopathologic findings, occurred within one month after transplantation in five recipients of kidney transplants from HLA-A, B and D identical sibling donors. In all cases, unidirectional mixed lymphocyte cultures and immunologic studies to detect cytotoxic antibodies in the recipients against their respective donors, before kidney transplantation and after transplant nephrectomy, were unresponsive or negative. Onset of delayed hyperacute rejection was preceded by bacteremia in two of these patients. Two of these received second kidney transplants, three to six months later, from HLA-A, B and D identical sibling donors again. Although both have had an episode of acute rejection in the early postoperative period, the grafts have maintained excellent function for 21 and 25 months, respectively. Irreversible forms of transplant rejection, such as delayed hyperacute rejection, do occur even in recipients of kidney transplants from HLA-A, B and D identical sibling pairs, indicating that genetic determinants other than HLA-A, B and D loci, and perhaps other nongenetic immune mechanisms, play an important role in the ultimate results of kidney transplantation.