The most frequent organ-specific autoimmune diseases associated with type 1 diabetes mellitus in children are hypothyroidism and celiac disease. Among adults, other associations exist, notably with pernicious anemia, which is extremely rare in children. We relate the observation of an adolescent with type 1 diabetes mellitus and hypothyroidism, admitted for severe anemia in addition to chronic anemia caused by autoimmune gastritis. Blood cell count showed severe aregenerative anemia with pancytopenia, with signs of non-autoimmune hemolysis. Vitamin B12 levels were low, bone marrow aspiration revealed erythroid hyperplasia, and anti-intrinsic factor antibodies were positive, providing the diagnosis of pernicious anemia. Treatment with intramuscular vitamin B12 produced brisk reticulosis after 6 days, with a subsequent rapid resolution of the anemia. Follow-up of type 1 diabetes mellitus in children requires screening for organ-specific autoimmune diseases; in case of unexplained anemia, autoimmune gastritis must be suggested. It can evolve into pernicious anemia.