Coeliac disease and type 1 diabetes mellitus can frequently coexist, presumably due to a common genetic predisposition. The present study was designed to evaluate the frequency of coeliac disease among Hungarian diabetic children and to study the effect of gluten-free diet on glycaemic control. A total of 205 diabetic children (age range 2.0-17.0 years, median 11.6 years) were screened for coeliac disease by determination of IgA-endomysium (EMA) antibodies. In the positive cases, a jejunal biopsy was performed and, in addition to routine histology, the number of intraepithelial gamma/delta T-cells was also determined. Insulin requirement, glycosylated haemoglobin level and body mass index of diabetic children with coeliac disease were determined before and 3 months after the introduction of gluten-free diet. IgA-EMA was positive in 24 cases, 17 of them (8.3% of all diabetic children) had a subtotal villous atrophy and thus coeliac disease was diagnosed. In all but two of these children, the mean number of gamma/delta T-cells was elevated (above 7 cells/mm). Of the remaining seven patients with positive EMA but normal villous structure, five (2.4%) had elevated number of epithelial gamma/delta T-cells, indicating probable latent coeliac disease. The insulin requirement of the children had significantly increased 3 months after the introduction of gluten-free diet (median values 0.64 versus 0.48 U/kg per day, P<0.05). Median body mass indices also showed significant elevation after this period (16.8 versus 14.2 kg/m(2), P<0.05) CONCLUSION: the frequency of coeliac disease was high in the studied group. Introduction of a gluten-free diet improved the somatic development of these children. A latent form of coeliac disease is also frequent in children with type 1 diabetes mellitus.