Previous studies have suggested an association between polymorphisms in the insulin gene region and insulin-dependent diabetes mellitus (IDDM). Most of the studies so far have been performed in Caucasoid populations. We have investigated 418 random IDDM patients and 422 healthy control subjects from three different ethnic groups; Tanzanian blacks, Norwegian Caucasians and Japanese orientals. Our data suggest that polymorphisms in the insulin gene region confer susceptibility to IDDM in Caucasians, and that a similar tendency though not statistically significant is observed among Tanzanian blacks, while no significant contribution is seen among Japanese orientals. We further demonstrate that the disease-associated genotype INS +/+ confers susceptibility independently of HLA class II alleles associated with IDDM. Compared to the contribution of particular HLA-DQ alleles in IDDM susceptibility, the additional risk conferred by the insulin gene region polymorphism is, however, small. Genotyping of the insulin gene region will therefore most probably not be a useful tool in the prediction of IDDM.