Type 1A diabetes mellitus has become one of the most intensively studied autoimmune disorders, with characterized animal models and extensive prospective studies of the development of anti-islet autoimmunity. It is now possible to predict the development of type 1A diabetes mellitus, beginning with HLA-encoded genetic susceptibility, followed by the development of a series of anti-islet autoantibodies. Prediction primarily is based on the detection of multiple anti-islet autoantibodies reacting with cloned islet antigens. Multiple international workshops fostered the development of specific and sensitive radioassays for autoantibodies reacting with GAD65 (glutamic acid decarboxylase), ICA512 (also termed IA-2, a tyrosine phosphatase-like protein), and insulin. Similar high throughput radioassays have been applied using autoantigens for additional autoimmune disorders including celiac disease and Addison's disease. Relatives of patients with type 1A diabetes mellitus inherit susceptibility to express multiple autoantibodies, and a subset of autoantibody-positive individuals inherit susceptibility to progress to overt disease. This article reviews autoimmune disorders associated with type 1A diabetes mellitus.