One hundred four cases of clinically significant thyroid carcinoma (TC) occurred in a demographically well defined area with, on an average, 243,000 inhabitants, during an 18-year period, corresponding to a yearly incidence of 2.4 per 100,000. During the later years of the study there was an increase of the age-standardized incidence of differentiated TC. The reason for this is suggested to be a greater health awareness, because only the number of tumors with less advanced growth increased. Sixty-one patients had, as the only presenting sign, a solitary thyroid nodule, while 24 had obviously malignant disease. All cases were revised histologically. Sixty-six patients were found to have papillary carcinoma, whereas 22 cases were diagnosed as follicular, 4 as medullary, and 12 as anaplastic. The prognosis, as estimated by the life-table method, was worse for patients with anaplastic cancer followed by follicular, papillary, and medullary. Within the papillary group, patients with occult cancer, i.e., thyroid tumor not larger than 1.5 cm, and intrathyroidal cancer, i.e., thyroid tumor larger than 1.5 cm but not penetrating the thyroid capsule, had a cumulated survival rate not significantly different from the expected rate, and only 1 of the 46 patients belonging to these two subgroups, died from TC.