Background/aims: The objective of this study was to determine the prevalence of Celiac disease in Turkish children with type I Diabetes Mellitus and their non-diabetic first-degree relatives. Methods: Forty-eight children with type I Diabetes Mellitus (18 males, 30 females; age range: 3.5 to 23 years; mean age: 12.09 +/- 4.78 years), 29 non-diabetic siblings, 40 non-diabetic parents, and 103 healthy children were screened for celiac disease using the IgA and IgG anti-tissue transglutaminase antibody and total serum IgA. Small intestinal biopsy was offered to all antibody-positive patients. Results: Eight of 48 diabetic patients had positive anti-tissue transglutaminase IgA. Selective IgA deficiency was detected in 2 diabetic children and both were positive to anti-tissue transglutaminase IgG. Intestinal biopsy was accepted by 8 of 10 (80%) diabetic children with positive celiac serology. Pathologic examination showed total villous atrophy in 3 (6.3%) diabetic children. Positive anti-tissue transglutaminase IgA was found in 1/29 siblings and 2/40 parents. Celiac disease was confirmed by biopsy in the sibling. Two parents refused the biopsy. The frequency of biopsy-proven celiac disease was found as 1.4 in relatives of diabetic children. None of the serum samples of healthy children comprising the control group showed selective IgA deficiency or positivity for anti-tissue transglutaminase IgA antibody. Conclusions: These findings indicate that the prevalence of celiac disease in Turkish children with type 1 diabetes mellitus is higher than in healthy controls. The 1.4% frequency of Celiac disease in relatives of diabetic children is close to that of controls.