There is an increased prevalence (P less than 0.001) of IgA deficiency in children with juvenile-onset insulin-dependent diabetes mellitus (9/366) but not in adults with insulin-dependent diabetes (0/421). The juvenile diabetics with IgA deficiency have other immune-associated diseases, such as thyroiditis and chronic active hepatitis, and have a history of infections. Four of the nine IgA-deficient diabetics we studied have autoantibodies to endocrine organs. Seven of eight have the HLA-B8, a proportion significantly (P less than 0.05) greater than control populations. Based on the clinical findings of IgA deficiency and multiple autoantibodies in patients with ataxia-telangiectasia and chronic mucocutaneous candidiasis, diseases associated with thymus deficiency, we suspect that thymus deficiency and autoimmunity may play a role in the pathogenesis of some types of juvenile-onset diabetes mellitus. In addition, an excess morbidity of the IgA-deficient juvenile diabetic population may explain the lack of IgA deficiency in older insulin-dependent diabetic individuals.