OBJECTIVE To determine whether there was a significant increase in the incidence of thyroid carcinoma in Olmsted County, Minnesota, that may be attributed to the widespread use of therapeutic head and neck irradiation between 1920 and the 1950s or to exposure to atomic fallout at the Nevada Test Site in the 1960s. METHODS Rochester Epidemiology Project resources were used to identify potential cases of thyroid carcinoma among residents of Olmsted County between 1935 and 1984. We extended this earlier study through 1999. RESULTS During the study period, thyroid carcinoma was newly diagnosed in 263 residents. In women, the age-adjusted incidence increased from 2.7 per 100,000 person-years (p-y) in 1935-1949 to 9.2 per 100,000 p-y in 1990-1999 (P=.001); in men, the rate increased from 0.8 to 5.0 per 100,000 p-y (P=.007). However, most of the increase occurred before 1965 and remained relatively stable thereafter. Similar trends were observed for papillary carcinoma alone. CONCLUSION Although the incidence of thyroid carcinoma increased significantly between 1935 and 1964, no significant increases have been seen since 1965, suggesting that neither atmospheric atomic fallout from the Nevada Test Site nor use of ionizing radiation to treat conditions of the head and neck significantly affected the incidence of thyroid carcinoma in Olmsted County.