It has been postulated that variation in the distribution of human leukocyte antigen (HLA)-encoded susceptibility alleles for insulin-dependent diabetes mellitus (IDDM) is the genetic basis for variation in the incidence of the disease between populations. The aim of this study was to characterize HLA-encoded susceptibility to IDDM in Hungary and to identify whether HLA-DRB1/DQ-encoded susceptibility could account for the five times lower incidence of disease in Hungary compared with Finland. The haplotypes DRB1*03-DQA1*05-DQB1*02 (DRB1*03-DQ2) and DRB1*04-DQA1*0301-DQB1*0302 (DRB1*04-DQ8) were significantly associated with disease in both populations. Three genotypes incorporating either or both of these haplotypes accounted for over 70% of the diabetic population in both races. The combined background frequency and the degree of risk as measured by odds ratios of these HLA-DRB1-DQ genotypes were not significantly different in the two countries. Comparison of the DRB1*0401-DQ8 haplotype between the two races suggested a role for HLA-B alleles in susceptibility. These data indicate that the susceptibility associated with high risk DRB1-DQ genotypes alone is insufficient to account for the fivefold variation in incidence of IDDM between Hungary and Finland. Other genetic and/or environmental influences must be involved.