In 2347 patients fine needle aspiration of the thyroid gland was performed. In 49 cases (2.1%), a final diagnosis of thyroiditis was established. Non-specific granulomatous thyroiditis occurred most frequently (n = 24), followed by lymphocytic hypertrophic thyroiditis Hashimoto (n = 18), focal lymphocytic (n = 5), atrophic lymphocytic (n = 1) and chronic fibrosing thyroiditis (n = 1). Fine needle aspiration biopsy is most suited to diagnose granulomatous thyroiditis. In Hashimoto's thyroiditis cytological investigations are superior to estimation of thyroglobulin antibodies. The cytological method is of little value for the diagnosis of atrophic lymphocytic and chronic fibrosing thyroiditis.