Serial thyroid functions studies were carried out in patients with melanoma and renal cell carcinoma treated with interleukin-2 (3 MU m-2 by continuous infusion days 1-4) and interferon alpha-2a (6 MU m-2 subcutaneously on days 1 and 4), both given on alternate weeks. The results on eight patients who completed at least three cycles of treatment are described. Four patients developed thyroid dysfunction with a hyperthyroid phase of 2 weeks followed by a hypothyroid phase ranging from 12 to 24 weeks. Two patients became clinically symptomatic and required treatment. Fine-needle aspirates of the thyroid were obtained in three patients with thyroid dysfunction. The cytology revealed a mixed cellular infiltrate with lymphocytes and histiocytes, and immunocytochemical staining showed strong HLA-DR expression of all thyrocytes, both suggestive of an autoimmune thyroiditis. One patient with thyroiditis developed anti-thyroglobulin antibodies, the serology of all other patients was normal. Patients with thyroid dysfunction tended to have higher in vivo stimulated lytic activity of peripheral mononuclear blood cells and had significantly higher levels of CD16 positive blood cells as compared to euthyroid patients. The possibility of autoimmune thyroiditis should be anticipated in future trails combining interleukin-2 and interferon alpha-2a.