As suggested from clinical data and on the basis of human leukocyte antigen (HLA) data, insulin-dependent diabetes mellitus (IDDM) is a disease entity in itself and is different from non-insulin-dependent diabetes and other types of diabetes mellitus in aetiology and pathogenesis. HLA-B8 is associated with IDDM in all Caucasian populations studied, irrespective of the age of onset of the disease. HLA-B15 is associated with IDDM in populations of Northern European and British origin, while B18 seems to replace B15 in Southern European populations. IDDM is uncommon in populations where the HLA-B8 frequency is low, and in the Japanese IDDM occurs in association with Bw22. The HLA-Dw3 and Dw4 association with IDDM is stronger than that of the B alleles. Relative risks for B8 and B15 heterozygous and homozygous individuals are identical, i.e., no gene-dose effect exists. The relative risk of B8/B15 carriers is double that of relative risks of B8 and B15 alone, i.e., there are two IDDM-associated genes. The same applies to Dw3/Dw4 carriers. In families the phenotype IDDM segregates with a certain genotype, the diabetic proband's HLA haplotype. Only a small proportion of family members carrying the 'diabetic haplotype' develop IDDM.