Sera obtained at diagnosis from 273 children (0-14 years) with insulin-dependent diabetes mellitus (IDDM) were studied to compare different autoantibody levels. The subjects comprise 75 % of all incident cases in New South Wales, Australia, for a 2-year period (ascertainment > 99% complete). Antibodies against glutamate decarboxylase were measured by radioimmunoprecipitation, insulin autoantibodies (on 176 sera collected within 4 days of initiation of insulin therapy) by radioimmunoassay, thyroid peroxidase and antigliadin IgA antibodies by enzyme-linked immunoassay, and anti-endomysial IgA and islet cell antibodies by indirect immunofluorescence. Reference ranges for anti-glutamate decarboxylase and insulin autoantibodies were determined in a group of non-diabetic children. Of the sera 69 % were positive for anti-glutamate decarboxylase, 65 % for insulin autoantibodies, 71% for islet cell antibodies (>/20 Juvenile Diabetes Foundation units), 10 % for anti-thyroid peroxidase, 2.6 % for antigliadin and 3.0 % for anti-endomysial antibodies. Islet cell antibodies and insulin autoantibodies were both negative in 13.7 % of the sera, while only 5.8 % were negative for all three of islet cell antibodies, insulin autoantibodies and anti-glutamate decarboxylase. There was a higher frequency of anti-glutamate decarboxylase among girls than boys (75 % vs 63 %, p = 0.03) and a negative correlation between the level of insulin autoantibodies and age at diagnosis (r =-0.41, p < 0.0001). A higher frequency of antithyroid peroxidase was found with increasing age (p = 0.05). Higher titres of islet cell antibodies were associated with a higher frequency of both anti-glutamate decarboxylase (p < 0.0001) and insulin autoantibodies (p = 0.003). Five children (1.8 %) with clear elevations of antigliadin and anti-endomysial antibodies were found to have asymptomatic coeliac disease by small bowel biopsy. [Diabetologia (1994[Diabetologia ( ) 37: 1113[Diabetologia ( -1120] ] Key words Autoantibodies, glutamate decarboxylase, islet cell antibodies, insulin, thyroid peroxidase, coeliac disease.