A 36-year-old man with a chronic hepatitis C infection developed hyperthyroidism from a previously normal thyroid status. Laboratory tests showed both elevated serum thyroxine and anti-thyroid antibody titres. The patient's hyperthyroidism was brought into remission by the administration of anti-thyroid medication. Graves' disease is a possible autoimmune manifestation of chronic hepatitis C not associated with interferon therapy. The mechanism may be due to antigen mimicry, epitope modification, or T-cell activation by the hepatitis C virus. New treatment modalities may be more effective than interferon therapy for chronic hepatitis C patients with thyroid disorders.