Type 1 diabetes mellitus is an autoimmune disease in which the presence of different autoantigens can often be found. The aim of our study was to evaluate the prevalence of antibodies against insulin (IA) and autoantibodies against glutamic acid decarboxylase (anti-GAD), tyrosine phosphatase IA-2 (anti-IA-2), thyroid microsomal peroxidase (anti-TPO) and thyroglobulin (anti-TG) in 55 randomly selected Type 1 diabetic patients (34 males, 21 females). Mean age of these patients was 39 +/- 12 yrs, mean duration of diabetes 18 +/- 13 yrs. Positivity of anti-GAD was found in 29 (58%) patients, anti-IA-2 in 13 (25%) patients, IA in 46 (85%) patients, anti-TPO in 10 (21%) and anti-TG in 11 (23%) patients. Simultaneous positivity of thyroid and islet autoantibodies was found in 6 (11%) patients whereas the positivity at least one of them was in 38 (69%) patients. No relationship between glycated hemoglobin and autoantibody concentration was found in the whole group of patients. The autoimmune thyroid disease was newly detected in 4 patients from high concentration of thyroid autoantibodies together with impaired TSH and T4 values and ultrasonography finding. No clinical evidence of thyroid disease was previously found in these patients. Positivity of anti-GAD or anti-IA-2 was found in almost 65% and of any thyroid autoantibody in almost 30% of our patients. Four patients with autoimmune thyroid disease were newly identified. We conclude that the evaluation of thyroid autoantibodies in Type 1 diabetic patients may improve the diagnosis of thyroid disease in very early stage and thus prevent consequent complications.