During the period 1935 to 1951, there were 283 newly diagnosed cases of thyroid cancer among residents of the state of Connecticut. The median survival after diagnosis for the 251 persons with histological confirmation of cancer was three and one-half years. At the end of the 12th month, two-thirds of the patients were still alive, whereas two-thirds of the members of a control group had not yet died by the end of the 12th year. Thyroid cancer progresses five times more rapidly in men than in women. Women with papillary cancer have a considerably better prognosis (median survival nine and one-half years) than women with the non-papillary variety (median survival two years). The survival of men was poor regardless of the histological variety of the neoplasm. For both sexes combined the median survival in localized thyroid cancer was four and one-half years for the nonpapillary types and eight years for the papillary variety.