Autoimmune diseases can be diagnosed early through the detection of autoantibodies. The aim of this study was to determine the risk of organ-specific autoimmunity in individuals with a family history of type 1 diabetes. RESEARCH DESIGN AND METHODS The study cohort included 2,441 first-degree relatives of patients with type 1 diabetes who were prospectively followed from birth to a maximum of 29.4 years (median 13.2 years). All were tested regularly for the development of autoantibodies associated with type 1 diabetes (islet), celiac disease (transglutaminase), or thyroid autoimmunity (thyroid peroxidase). The outcome was defined as an autoantibody-positive status on two consecutive samples. RESULTS In total, 394 relatives developed one (n = 353) or more (n = 41) of the three diseaseassociated autoantibodies during follow-up. The risk by age 20 years was 8.0% (95% CI 6.8-9.2%) for islet autoantibodies, 6.3% (5.1-7.5%) for transglutaminase autoantibodies, 10.7% (8.9-12.5%) for thyroid peroxidase autoantibodies, and 21.5% (19.5-23.5%) for any of these autoantibodies. Each of the three disease-associated autoantibodies was defined by distinct HLA, sex, genetic, and age profiles. The risk of developing any of these autoantibodies was 56.5% (40.8-72.2%) in relatives with HLA DR3/DR3 and 44.4% (36.6-52.2%) in relatives with HLA DR3/DR4-DQ8. CONCLUSIONS Relatives of patients with type 1 diabetes have a very high risk of organ-specific autoimmunity. Appropriate counseling and genetic and autoantibody testing for multiple autoimmune diseases may be warranted for relatives of patients with type 1 diabetes. Prospective studies that follow children from birth have established that autoimmunity can start very early in life and years before clinical symptoms appear (1-3). In type 1 diabetes, autoimmunity often appears between 6 months and 3 years of age (1-3), and the large majority of children who develop autoantibodies against multiple pancreatic islet antigens at this age progress to clinical disease (4). New disease paradigms have emerged from such studies, leading to extensive screening (5), the