OBJECTIVE: To estimate the prevalence of biopsy-confirmed celiac disease in Italian children and adolescents with type 1 diabetes and to assess whether age at onset of type 1 diabetes is independently associated with diagnosis of celiac disease. RESEARCH DESIGN AND METHODS: The study group was a clinic-based cohort of children and adolescents with type 1 diabetes cared for in 25 Italian centers for childhood diabetes. Yearly screening for celiac disease was performed using IgA/IgG anti-gliadin and IgA anti-endomysium antibodies. RESULTS: Of the 4,322 children and adolescents (age 11.8 +/- 4.2 years) identified with type 1 diabetes, biopsy-confirmed celiac disease was diagnosed in 292 (prevalence 6.8%, 95% confidence interval [CI] 6.0-7.6), with a higher risk seen in girls than in boys (odds ratio [OR] 1.93, 1.51-2.47). In 89% of these, diabetes was diagnosed before celiac disease. In logistic regression analyses, being younger at onset of diabetes, being female, and having a diagnosis of a thyroid disorder were independently associated with the risk of having diabetes and celiac disease. In comparison with subjects who were older than 9 years at onset of diabetes, subjects who were younger than 4 years at onset had an OR of 3.27 (2.20-4.85). CONCLUSIONS: We have provided evidence that 1) the prevalence of biopsy-confirmed celiac disease in children and adolescents with type 1 diabetes is high (6.8%); 2) the risk of having both diseases is threefold higher in children diagnosed with type 1 diabetes at age <4 years than in those age >9 years; and 3) girls have a higher risk of having both diseases than boys.