The authors followed up the incidence of permanent hypothyroidism in 35 patients with subacute thyroiditis (SAT) in their past history. The investigations were made one to four years after the acute episode. Ten of these presented repeated recurrences despite the therapy with glucocorticoids. The investigations included serum determination by RIA of T4, T3, TSH and thyroglobulin as well as of circulating thyroid antibodies. Permanent hypothyroidism subsequent to SAT occurred in two patients. In one of them it occurred following thyroid surgery performed for repeated recurrences. Both patients presented high levels of circulating thyroglobulin antibodies suggesting a possible transition to autoimmune thyroiditis. In the authors' opinion permanent hypothyroidism is likely to develop after SAT only in association with an autoimmune process or after thyroid surgery.