In order to obtain prospective data on occurrence of thyroid autoantibodies [against thyroid peroxidase (antiTPO) and against thyroglobulin (antiTgl)] and their clinical relevance, we followed up on 109 young adults with Type 1 diabetes for 12 yr after diabetes onset. The patients were divided into subgroup I [positivity of both thyroid autoantibodies (T-Ab); 25%, women predominantly], subgroup II (isolated antiTPO positivity only, 26%, men and women equally) and subgroup III (thyroid autoimmunity not present, 49%, men mainly). Cumulative incidence of T-Ab during the 12 yr of follow-up was 51% with predominance of women over men (65% vs 38%, p<0.01). At the time of the first T-Ab detection, an ultrasonography pattern of a hypoechogenic thyroid gland was noted in 59% of subgroup I patients and in 25% of subgroup II patients (p<0.05). At the same time, TSH>4.5 mlU/I was shown in 30 and 7% of patients of subgroups I and II, respectively (p<0.05). In the patients with the repeated positivity of both T-Ab (subgroup I) subclinical hypothyroidism developed in all patients within 4 yr after the first detection of T-Ab. On the contrary, the clinical course in patients with isolated antiTPO positivity (subgroup II) was milder with 11% developing subclinical hypothyroidism within 4 yr after the first antiTPO detection (p<0.001). The data shed new light on the relative diagnostic value of thyroid antibodies, ultrasonography and functional examination for an early detection of thyroid autoimmunity of adult diabetic patients.