Autoimmune thyroid disease is the result of a common side-effect of interferon-alpha (IFN-alpha) used to treat viral hepatitis C; but there have been few reports on thyroid disorders induced by IFN-a that was used to treat renal cell carcinoma. IFN-alpha therapy was conducted on two male patients, 75 and 44 years old, after radical nephrectomy. Six and five months, respectively, after this therapy, they complained of weight loss. Laboratory evaluation revealed hyperthyroidism; the thyroid stimulating hormone (TSH) level fell below normal; and the serum free T3 and T4 levels increased above normal values. Two months after the termination of IFN-alpha therapy, their thyroid hormone levels returned to normal without the help of antithyroid agents. In observational studies, thyroid dysfunction has been reported in 0.6 to 30% of the patients who had been treated with IFN-alpha. Careful observation is necessary to watch for the possible development of thyroid disorder during IFN-alpha therapy for renal cell carcinoma.