AbstractThe etiology of IDDM remains incompletely understood, with genetic predisposition, autoimmune destruction of the beta cells and viral infections interacting to produce disease. It appear that the underlying genetic defect is an alteration in cellular and/or humorally‐mediated immunological responsiveness. There are at least two (and probably more) genetically determined defects identified by HLA B‐8 (DR3) and B‐ 15 (DR4). A number of viruses have been implicated in IDDM causation, including Coxsackie, mumps, rubella, Epstein‐Barr, hepatitis and influenza viruses, but in only a few cases is viral etiology fully documented. Viral infections may trigger autoimmunity. Alterations in immunoglobulin concentration, particularly IgA deficiency, have been reported by several investigators. Collaborative prospective studies of individuals at high risk for the development of IDDM (siblings with HLA identical to the index case) are essential in the future study of IDDM etiology.