Celiac disease (CD) is a small intestinal disorder with overt malabsorption in the minority and with subclinical or atypical symptoms in the majority of patients. It is triggered by gluten and related cereal proteins in a unique genetic background (HLA-DQ2 or DQ8 and other unmapped genes). CD is characterized by a highly specific mucosal autoantibody response to tissue transglutaminase. In the intestine this enzyme creates antigenic neoepitopes in gluten peptides which are more efficiently presented to the immune system in the context of HLA-DQ2 or DQ8. Between 3% and 6% of patients with type 1 diabetes mellitus (DM) have (atypical) CD, and the prevalence of a variety of autoimmune diseases in patients with CD correlates with the time of gluten exposure, reaching 35% after 20 years. It is still unknown whether oligosymptomatic CD favors the development of type 1 DM and whether a gluten-free diet modifies the progression of DM in general. Apart from shared or adjacent HLA loci in both diseases, post-translational modification of potential autoantigens by enzymes such as tissue transglutaminase could play a role in the autoimmunity of type 1 DM.