We compared 26 HLA-A, B identical sibling kidney-transplant recipients followed for one to 10 years, with 104 HLA-A, B semi-identical kidney recipients from living, related donors to determine clinical differences. Graft-survival rates were significantly better in the HLA identical group at two years (85 per cent identical versus 53 per cent in semi-identical, P less than 0.005); patient-survival rates were high for both (96 per cent in identical and 87 per cent in semi-identical at two years, P less than 0.005). The incidence of complications was similar in HLA identical and semi-identical recipients. Nine of the 26 grafts in HLA identical recipients failed one week to eight years after transplantation. Rejection caused most of the graft failures. Recipients of HLA identical-sibling kidney transplants have a high patient and graft survival, but they also encounter many complications. Immunologic rejection occurs, even with negative mixed lymphocyte culture, suggesting the importance of donor determinants other than the HLAA, B and D other than the HLA-A, B and D.