Celiac disease (CD) is one of the most frequent autoimmune disorders occurring in Type 1 diabetes mellitus (T1DM). The prevalence of CD in T1DM varies from 3 to 16%, with a mean prevalence of 8%. The clinical presentation of CD in T1DM is classified as symptomless in approximately half of cases, but a more accurate analysis often discloses a wide array of symptoms suggestive of CD. Both T1DM and CD show the same genetic background and an abnormal small intestinal immune response with inflammation and a variable grade of enteropathy. Serological screening for CD should be performed in all T1DM patients by means of antibodies to tissue transglutaminase at T1DM onset. T1DM patients found to be celiacs must be treated by a gluten-free diet. Potential CD cases (especially when asymptomatic) should be kept on a gluten-containing diet with a careful clinical and antibody follow-up, since many of them will not develop villous atrophy.