Type 1 diabetes (T1DM) is associated with other autoimmune diseases (AIDs), which may have serious health consequences. The epidemiology of AIDs in T1DM is not well defined in adults with T1DM. In this cross-sectional cohort study, we sought to characterize the incident ages and prevalence of AIDs in adults with T1DM across a wide age spectrum. RESEARCH DESIGN AND METHODSA total of 1,212 adults seen at the Washington University Diabetes Center from 2011 to 2018 provided informed consent for the collection of their age, sex, race, and disease onset data. We performed paired association analyses based on age at onset of T1DM. Multivariate logistic regression was used to evaluate the independent effects of sex, race, T1DM age of onset, and T1DM duration on the prevalence of an additional AID. RESULTSMean 6 SD age of T1DM onset was 21.2 6 14.4 years. AID incidence and prevalence increased with age. Female sex strongly predicted AID risk. The most prevalent T1DM-associated AIDs were thyroid disease, collagen vascular diseases, and pernicious anemia. T1DM age of onset and T1DM duration predicted AID risk. Patients with late-onset T1DM after 30 years of age had higher risks of developing additional AIDs compared with patients with younger T1DM onset. CONCLUSIONSThe prevalence of AIDs in patients with T1DM increases with age and female sex. Later onset of T1DM is an independent and significant risk factor for developing additional AIDs. Individuals who are diagnosed with T1DM at older ages, particularly women, should be monitored for other autoimmune conditions.Type 1 diabetes (T1DM) is a common autoimmune disease (AID) that affects at least 30 million people worldwide (1,2). Its rising incidence is driven by the interplay between individual genetics and environmental triggers (3,4). T1DM is characterized by autoimmune destruction of pancreatic islet b-cells, resulting in insulin deficiency and necessitating lifelong hormone replacement therapy. Classically described as a disease of childhood, T1DM is increasingly diagnosed in adults (5-7), and with longer life expectancy for all patients with T1DM, the overall prevalence of T1DM in adults has risen substantially. Although tremendous attention is devoted to screening for and management of diabetes-related microvascular and macrovascular complications in T1DM, less attention has been paid to the characterization of AIDs and related