It has now become clear that certain HLA antigens are associated with disease susceptibility more than any other genetic markers. Insulin-dependent diabetes mellitus (IDDM or type I) is an HLA-associated condition. Moreover, there is evidence to show that IDDM is a genetically programmed autoimmune disease. Studies of the HLA-DR region have shown a strong association with IDDM, with over 90 per cent of IDDM patients possessing DR3 and/or DR4. Although the HLA-DR region is a major component in the inherited disease susceptibility, it is not the only gene region involved. Recent studies demonstrated that HLA-DQ may be more closely linked to the disease locus than HLA-DR. Sequence analysis of the HLA-DQ3 gene products suggest that a single amino acid (aspartic acid) at position 57 is uniquely important for determining susceptibility or resistance to IDDM. Although there is a strong association of certain HLA loci with IDDM, it may not explain nor account for all the genetic susceptibility to the disease. It seems that 60 per cent of the genetic basis of IDDM is related to the HLA gene (chromosome 6) and another 40 per cent is non-HLA-associated (i.e., chromosomes 2, 7, 11, and 14). Even though great progress has been made in the understanding of the genetics of IDDM, the mode of inheritance of the disease remains controversial. The present review discusses various aspects of the autoimmune process believed to be involved in pancreatic beta cell destruction in individuals genetically susceptible to IDDM. The possible modes of inheritance and new data regarding estimated risks of transmitting the disease are presented.