Aims/hypothesis Coeliac disease and transglutaminase antibodies are common in patients with type 1 diabetes and their relatives. We investigated the development of transglutaminase antibodies and analysed potential risk factors for coeliac disease autoimmunity in first-degree relatives of patients with type 1 diabetes. Methods The study was conducted by prospective observational follow-up from birth of 1,511 children at increased risk of type 1 diabetes or coeliac disease born in Germany between 1989 and 2000. Mean follow-up was to age 7.6 years. Children were tested for transglutaminase and islet autoantibodies. Children were classified as transglutaminase antibody-positive if antibodies were detected by both ELISA and radiobinding assays. Results The risk of developing transglutaminase antibodies was 4.9% by age 8 years (n=63; 95% CI, 3.7-6.1%). Transglutaminase antibodies developed at an older age than islet autoantibodies (median age, 4.9 vs 2.3 years; p<0.005), and only three children developed both transglutaminase antibodies and islet autoantibodies. Multivariate analysis indicated an increased risk of transglutaminase antibodies in children with the HLA-DRB1*03 allele (hazard ratio for heterozygous DR3, 5.5; 95% CI, 2.9-10.2; hazard ratio for homozygous DR3, 16.2; 95% CI, 6.7-39; p<0.0001) and in children with impaired uterine growth (birth weight ≤ 1st percentile, hazard ratio, 3