Insulin dependent diabetes mellitus type I. (IDDM) is often connected with other autoimmune diseases. The most frequently concomitant disease is autoimmune thyroiditis. The authors present the results of the observation of 33 patients. The age of patients at the beginning of the study was 39.5 (24-65) years (median + confidential intervals). In all these patients, there were performed the assessment of thyroid stimulating hormone (TSH), free thyroxine (fT4), thyroid peroxidase antibodies (AbTPO) and TSH receptor antibodies (TRAK). An ultrasound examination and percutaneous aspiratory biopsy were performed in all patients. The control examination was realised 5 years later. In 1991, primary hypothyroidism due to lymphoid thyroiditis in 12.1% and autoimmune hyperthyroidism in 3% of all patients were found. During the observation period 2 patients died. In 1996 the diagnosis of primary hypothyroidism in 9.6% and M. Graves-Basedow in 3% of all patients were confirmed. Thyroid peroxidase antibodies have been positive in 33.4% of all patients with primary hypothyroidism and they correlated with ultrasound and cytology findings. (Tab. 3, Ref. 13.)