The prevalence of thyroid cancer is not nodules is low, as it is estimated to be between 0 and 4% in adults. In order to more accurately estimate this prevalence and to recommend optimal treatment, 93 hot nodules operated between 1976 and 1995 were reviewed. 52 of these subjects (56%) were euthyroid, 11 (12%) suffered from clinical hyperthyroidism and 30 (32%) a presented subclinical hyperthyroidism. All patients were operated for a hot nodule, 1.5 to 6 cm in diameter. Two groups of patients were considered on the basis on the histological examination. Group I consisted of 47 patients (39 F, 8 M, mean age: 44 years +/- 15) with multinodular goitre; 16 (34%) of them underwent total thyroidectomy. Group II was composed of 46 patients (38 F, 8 M, mean age: 44 years +/- 15) with a solitary hot nodule; 39 (85%) patients underwent unilateral lobectomy. Microscopic carcinoma (diameter less tha 1 cm) ws discovered in 2(4%) patients of group 1 and 5 (11%) patients of group 11, corresponding to a total prevalence of 7/93 (7.5%). The microscopic carcinoma was located in the nodule or in the capsule in four cases, and away from the nodule but in the ipsilateral lobe in three cases. In every case, this was an incidental finding and no clinical features distinguished patients with or without microscopic carcinoma. This study suggests that the probability of discovering a microscopic carcinoma associated with a hot nodule is considerable. However, microscopic carcinoma is an incidental discovery in a number of subjects undergoing thyroidectomy and is present in 10 to 20% of autopsied thyroids. As surgical treatment is devoid of risks, it appears indicated, particularly when the nodule exceeds 3 cm in diameter.