OBJECTIVE: Autoimmune thyroid disease (AIT), celiac disease, and Addison's disease are characterized by the presence of autoantibodies: thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TGAb) in AIT, tissue transglutaminase antibody (TTGAb) in celiac disease, and 21-hydroxylase antibody (21-OHAb) in Addison's disease. The objective of this study was to define the prevalence of these autoantibodies and clinical disease in a population with type 1 diabetes. RESEARCH DESIGN AND METHODS: We screened 814 individuals with type 1 diabetes for TPOAb, TGAb, TTGAb, and 21-OHAb. Clinical disease was defined by chart review. Factors related to the presence of autoimmunity and clinical disease including age at onset of type 1 diabetes, duration of diabetes, age at screening, sex, and the presence of autoantibodies were reviewed. RESULTS: The most common autoantibodies expressed were TPOAb and/or TGAb (29%), followed by TTGAb (10.1%) and 21-OHAb (1.6%). Specific HLA DR/DQ genotypes were associated with the highest risk for expression of 21-OHAb (DRB1*0404-DQ8, DR3-DQ2) and TTGAb (DR3-DQ2- DR3-DQ2). The expression of thyroid autoantibodies was related to 21-OHAb but not to TTGAb. The presence of autoantibodies was associated with and predictive of disease. CONCLUSIONS: In this large cohort of individuals with type 1 diabetes, the expression of organ-specific autoantibodies was very high. The grouping of autoantibody expression suggests common factors contributing to the clustering.