Serotyping and genotyping (polymerase chain reaction with sequence-specific oligonucleotide probes method) were conducted on 520 unrelated individuals to determine the linkage disequilibrium of HLA-B and HLA-DRB1. Analyses of 511 kidney transplants (300 related and 211 cadaver recipients) were carried out at 4 transplant centers using the linkage disequilibrium of HLA-B and HLA-DRB1 established previously. All transplant recipients received CsA immunosuppression and were transplanted from June 1983 to December 1991. There were 51 significant linkages formed between HLA-B and HLA-DRB1 alleles (P<0.05). DRB1-compatible transplants experienced a comparable 5-year graft success rate of 94% as did the HLA-identical recipients with a 100% 5-year success rate. However DRB1-incompatible recipients displayed a significantly reduced 5-year graft survival rate of 73% (73% vs. 94% P<0.01). The 5-year graft survival rate of HLA-DR-incompatible recipients of 71% was compatible to the 73% for HLA-DRBl-incompatible recipients. No variation of rejection rate for DRB1-compatible grafts was seen in any of the 4 transplant centers. The results also indicated that HLA-DRB1 compatibility was essential for optimal success rate, regardless of HLA class I mismatches. The overall conclusion was that matching for HLA-DR was important to achieve optimal kidney graft survival on the molecular level but not on the serotyping level.