Background & Aims: The relationship between celiac disease and many autoimmune disorders has been explained by the sharing of a common genetic factor. In a multicenter national study, we examined the relationship between the prevalence of autoimmune disorders in celiac disease and the duration of exposure to gluten. Methods: Over a 6-month period, 909 patients with celiac disease (group A; mean age, 16.1 ± 3.8 years; grouped according to age at diagnosis into three subgroups [group A1, <2 years; group A2, 2-10 years; and group A3, >10 years]), 1268 healthy controls (group B; mean age, 20.8 ± 4.5 years), and 163 patients with Crohn's disease (group C; mean age, 28.8 ± 10 years) were evaluated for the presence of autoimmune disorders. Results: Prevalence of autoimmune disorders in group A was significantly higher than in group B (14% vs. 2.8%; P < 0.000001) but not higher than in group C (12.9%). Prevalence of autoimmune disorders in celiac disease increased with increasing age at diagnosis: 5.1% in group A1, 17% in group A2, and 23.6% in group A3 ( P = 0.000001). In group A3, the prevalence of autoimmune disorders was significantly higher than in group C. In a logistic regression model, age at diagnosis was the only significant predictor variable of the odds of developing an autoimmune disorder ( r = 0.3; P < 0.000001). Conclusions: Our data show for the first time that the prevalence of autoimmune disorders in celiac disease is related to the duration of exposure to gluten.