DNA-polymerase chain reaction(PCR)-RFLP(restriction fragment length polymorphism) typing of the DQA1 gene was performed on 39 patients with type 1 diabetes and 30 controls. Analysis of the frequency of the subtypes of DQA1 alleles, using the DNA-PCR-RFLP typing technique, showed that the DQA1*0301 subtype was most strongly associated with the disease (97.4% vs 56.7%, R.R. = 19.8, pc less than 0.00005). These results indicated that DQA1*0301 may determine the disease susceptibility in the Japanese. In addition, we analyzed the frequency of the subtypes of DQA1 genes in the ten patients carrying the DRW8-DQW8 haplotype, and found that at least eight of them (80%) were classified as having DQA1*0301. The DRW8-DQW8-DQA1*0301 may be one of the susceptible haplotypes among the Japanese.