The incidence of thyroid cancer was examined temporally and geographically by age and sex from data provided by tumor registries in the United States and abroad. The temporal trends in Connecticut showed an increase in annual incidence after 1945, with an especially sudden increase in incidence in females. The increase occurred predominantly in older males and younger females. The increase in young females was confirmed by cohort analysis. The rates rose with age in both sexes, but recently females have developed a secondary peak in the fourth decade of life. The same phenomenon was observed in other U.S. data but not as clearly in data from ten foreign registries. These observations are consistent with the hypothesis that X-radiation therapy for benign conditions of the head and neck in childhood was a factor in the increased incidence of thyroid cancer in U.S. females, but some other etiologic or modifying factor should be sought to explain the increased incidence in U.S. males.