Several factors may explain the high incidence of carcinoma in these studies, which is not in line with our results or with common clinical experience. First, all of the previous studies retrospectively examined patients with Graves disease who were surgically treated, and the results deserve to be examined in some detail. In the earliest reports, thyrotoxicosis was considered insurance against thyroid cancer, a conclusion based mainly on the results of Beahrs et al25 and Sokal,26 who reported incidences of carcinoma in patients with Graves disease of 0.5% and 0.15%, respectively. Shapiro et al1 and subsequent reports2- 9 indicated that the coexistence of Graves disease and thyroid carcinoma was not rare. The differences between studies carried out in different decades probably reflect to some degree the changing criteria in the selection of patients with Graves disease for thyroidectomy. In the 1950s, thyroidectomy was the usual therapy for Graves disease; thereafter, a tendency toward medical and radioiodine therapy emerged, causing a progressive decline in the number of patients referred for thyroidectomy.27 Thus, in recent years, patients with Graves disease who receive surgical treatment constitute a select group with more serious forms of the disease and are not representative of the whole population of patients with Graves disease. Second, in some studies a number of patients with carcinoma had been previously treated with external radiation.1- 3 Third, the high frequency of thyroid carcinoma in the surgical series of patients with Graves disease is probably caused by occult and clinically inconsequential cancers uncovered at thyroidectomy. The difference between pathologically and clinically identified thyroid carcinomas is well known. While thyroid microcarcinomas are found in 5% to 36% of adults at autopsy, clinically detectable thyroid carcinomas constitute less than 1% of all human cancers, while the annual incidence rate in various parts of the world ranges from 0.5 to 10 cases per 100,000.28 The possibility of a bias related to the pathological examination of the excised thyroid is supported by studies in which the incidence of carcinoma can be compared between patients with Graves disease who have undergone thyroidectomy and those who have not undergone surgery. Behar et al3 found thyroid carcinoma in 10 (5.2%) of 194 patients who underwent thyroidectomy, but only 1 (0.3%) of 303 patients treated with radioiodine therapy developed thyroid carcinoma during a 9-year follow-up. Pacini et al4 found thyroid carcinoma in 6 (7%) of 86 patients with Graves disease who underwent thyroidectomy, but among 1137 patients treated with methods other than surgery, only 1 developed thyroid carcinoma. In the same study, 4 (22%) of 18 patients with Graves disease and palpable thyroid nodules had carcinoma, while none of the 147 patients who did not undergo surgery developed thyroid carcinoma during 15 years of follow-up.