The increased prevalence of coeliac disease (CD) among children with type 1 diabetes mellitus (DM1) implies that there is more than a simple association. A link between the gut immune system and DM1 has been suggested both in animal models and in humans. We review the literature on the epidemiology and genetic and clinical aspects shared by these two diseases and speculate on the role of gluten on the possible relationship between CD and DM1, on the basis of recent animal and human studies. The data suggest a failure in oral tolerance mechanisms in DM1 other than that in CD. It remains to be understood why only a small proportion of patients with DM1 proceed to the production of coeliac-associated antibodies and to overt enteropathy.