The incidence of thyroid carcinoma in hyperthyroidism varies considerably from as low as 0.3% to as high as 16.6% with a higher rate in toxic nodular goiters. 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 131 I scan. In 408 patients who underwent surgery for hyperthyroidism in our Surgery Department from January 1967 through December 1994 the incidence of thyroid carcinoma was 5.6% (23 cases). In detail, a neoplasm occurred in 5 cases of Graves' disease (specific incidence: 3.8%), in 13 cases of toxic nodular goiter (12.5%) and in 5 cases of hyperfunctioning adenomas (2.8%). 19 cancers were papillary (12 in toxic nodular goiter, 3 in Graves' disease, 4 in hyperfunctioning adenomas), three were follicular (1 in Graves' disease, 1 in toxic nodular goiter, 1 in hyperfunctioning adenomas) and 1 medullary in Graves' disease. A papillary carcinoma was diagnosed preoperatively on fine needle aspiration with ultrasonography in only two patients with Graves' disease and confirmed by postoperative histological examination on permanent section. We do not believe in the frozen-section examination intraoperatively because it's not diagnostical for follicular lesions and evaluates rarely capsular invasion. Twenty patients received total thyroidectomy and four of them also lymphoadenectomy. Three patients received emithyroidectomy: in two cases for occult papillary carcinoma and in the last case for local cancer invasion (T4N0M0). Twenty patients are alive and with no evidence of cancer recurrence. Mean follow-up is 59.6 months. Our retrospective study shows a progressive increase of the incidence of coexisting thyroid malignancy and hyperthyroidism especially in toxic nodular goiter, probably related to extended surgical indications. Our findings do confirm that, even in the presence of hyperthyroidism, all thyroid nodules require careful diagnostics for exclusion of malignancy.