Jejunal biopsy was performed without selection in 110 of 201 children with insulin-dependent diabetes mellitus; serum reticulin antibody, antigliadin antibody by enzyme-linked immunosorbent assay, and serum IgA were studied in all 201 children. Seven children had severe jejunal villous atrophy, giving a prevalence of celiac disease of at least 3.5%. Of the serum tests used, antigliadin antibody with ELISA was the most sensitive. Four patients adhered to a gluten-free diet, and their jejunal structure became normal; three had subsequent gluten provocation, and the jejunal mucosa relapsed in every one. Six had HLA-B8 and -DR3 antigens, and one had B15 and -DR4 phenotypes. In most patients, a gluten-free diet had little effect on insulin dosage, urinary excretion of glucose, or serum level of hemoglobin A1.