The gene frequencies and haplotypic associations within the HLA region have been investigated in 916 unrelated Japanese individuals. HLA class I and class II antigens were studied by conventional serology, and class II alleles, DRB1, DRB3, DQA1, DQB1 and DPB1 were typed by using polymerase-chain reaction amplification and sequence-specific oligonucleotide probe (PCR-SSOP) method. Thirty DRB1, 3 DRB3, 8 DQA1, 15 DQB1 and 13 DPB1 alleles were found in our population. DR-NJ25, a characteristic antigen in the native American and Asian populations, was observed at 3.0%. This antigen was observed mainly with the DRB1*1403 and 1406 alleles. Twenty-seven out of 30 DRB1 alleles found in this study had a high positive linkage disequilibrium with DQB1 alleles and 20 of them had an exclusive association with one specific DQA1-DQB1 combination. The strong association between DRB1 alleles and HLA-B antigens was the most striking finding in this study. Twenty-eight out of 30 DRB1 alleles had a positive linkage disequilibrium with 24 HLA-B antigens (p < 0.01). The other two alleles, DRB1*0404 and 1402, were very rare, and their frequencies were 0.2% and 0.1%, respectively. The data presented in this population study should be useful for the studies on anthropology, organ transplantation and disease susceptibility.