||The relationship between hyperthyroidism due to Graves' disease or toxic nodular goiters and carcinoma of the thyroid remains uncertain. The incidence of thyroid carcinoma varies considerably from as low as 0.3% to as high as 16.6% with a higher rate in toxic nodular goiters than in diffuse goiters and in endemic goiter areas or after external radiation to head and neck. Occult thyroid carcinoma ( < 1.5 cm or microscopic foci) is the rule and only a few tumors are suspected preoperatively with ultrasonography or fine needle aspiration or 131I scan. Authors studied 11 patients found to have thyroid cancer concomitant with Graves' disease (2 patients) or toxic nodular goiter (9 patients) between 1979 and 1991 at University of Cagliari, Department of Surgical Oncology. Eight of the 11 cancers were papillary while three were follicular. A carcinoma was diagnosed intraoperatively on frozen section in only two patients, while on permanent section in nine patients. Two patients received subtotal thyroidectomy, because resection edges were safe and tumors were small. Nine patients received total thyroidectomy and three also lymphoadenectomy. All patients were alive and well with a mean follow-up of 91.8 months. The authors conclude that the incidence of coexisting thyroid malignancy and hyperthyroidism is rare, does not affect prognosis and is unimportant for therapeutic purposes because, currently, toxic nodular goiter, Graves' disease and carcinoma are treated by total thyroidectomy.