Type 1A diabetes is an autoimmune disease with genetic and environmental factors contributing to its etiology. Twin studies, family studies, and animal models have helped to elucidate the genetics of autoimmune diabetes. Most of the genetic susceptibility is accounted for by human leukocyte antigen (HLA) alleles. The most-common susceptibility haplotypes are DQA1* 0301-DQB1* 0302 and DQA1* 0501-DQB1* 0201. Less-common haplotypes such as DQA1* 0401-DQB1* 0402 and DQA1* 0101-DQB1* 0501 are associated with high risk for diabetes; however, large study populations are needed to analyze their effect. The DQA1* 0102-DQB1* 0602 haplotype is associated with diabetes resistance. DR molecules, such as DRB1* 1401, confer protection from diabetes. Monozygotic twins of patients with type 1A diabetes have a diabetes risk higher than that for HLA-identical ordinary siblings, suggesting that non-HLA genes contribute to diabetes risk. Polymorphisms in the regulatory region of the insulin gene (designated IDDM2), polymorphisms in cytotoxic T lymphocyte antigen-4 (CTLA-4) gene (IDDM12), and other genes are likely to contribute to diabetes risk and susceptibility in some individuals. In selected families, major diabetogenes (eg, IDDM17, autoimmune regulator gene (AIRE)) are likely to be of importance. Other factors--either noninherited genes (ie, somatic mutations and T-cell receptor or immunoglobulin rearrangements) or environment--may have a role in progression to diabetes. This is suggested by the finding that the risk for monozygotic twins of patients with type 1A diabetes is not 100 percent. Studying the genetics of type 1A diabetes will allow us to better define this disease, to improve our ability to identify individuals at risk, and to predict the risk of associated disorders.