All cases of thyroid cancer diagnosed in Finland in 1958-62 according to the Finnish Cancer Registry were re-examined and re-classified histologically. All the carcinomas in which papillary structures were seen regardless of the presence of follicular or solid areas as well, and also the rare purely follicular tumors with so-called ground-glass nuclei were classified as papillary carcinoma. The follicular carcinomas comprised the non-papillary carcinomas in which follicular or trabecular structures were seen and in which the nuclei were not of ground-glass type. There were 100 papillary and 60 follicular carcinomas. The histologic types differed in age and sex distribution, course of the disease and survival. The female:male ratio of the age-adjusted incidence rates was 3.3 for papillary but only 1.2 for follicular carcinoma. Papillary carcinoma occurred in all age groups, but the follicular type only in middle and old age. Regional lymph node metastases were common (42%) and distant metastases rare (14%) in papillary carcinoma. The situation was reversed in follicular carcinoma (2% and 72%). The survival rates were significantly higher for papillary than for follicular carcinoma. The relative 5-year rates were 83% and 54%. Papillary and follicular carcinoma appear to be two biologically different tumors with no intermediate or mixed forms. Therefore, they should be distinguished in clinical and in scientific work.