From a study of 3,171 related donor kidney transplants several points of interest emerged: 1. HLA-identical sibling transplants survive at a rate of approximately 85% at 1 year, regardless of whether two, three, or four HLA antigens are identified. 2. Offspring donor transplants survive at similar rates, regardless of whether the recipient is a mother or a father. 3. Transplants from uncles or aunts survive at similar rates as parental donor transplants. 4. Non-HLA antigens contribute approximately 15% to the 1-year kidney transplant failure rate. 5. The strength of an HLA haplotype is reflected in a difference of approximately 15% in the 1-year graft survival rate. 6. The strength of a single HLA antigen contributes about 4% to the 1-year graft survival. 7. The HLA-A or -B loci and antigens of different HLA specificities are roughly of equal immunogenicity. 8. The relative strength of HLA antigens in human kidney transplants is best shown in the well controlled situation of related donor grafts.