The incidence of second primary tumors was studied through record-linkage in 2,968 thyroid cancer patients reported to the Swedish Cancer Registry during the period 1958-1975. The cohort was matched with the Swedish Cancer Registry between 1959 and 1984. A total of 283 second primary tumors were reported more than one year after thyroid cancer diagnosis, and the standardized incidence ratio (SIR) was 1.18 (95% confidence interval = 1.03-1.31). A significant elevated risk of cancer of the kidney, endocrine glands, and nervous system was noted. Men had a higher risk (SIR = 1.37; 95% CI = 1.06-1.70) than women (SIR = 1.11; 95% CI = 0.96-1.28). Patients who were 36-45 years at the time of the thyroid cancer diagnosis were at highest risk of developing a second primary tumor (SIR = 1.35; 95% CI = 0.99-1.81). Significantly elevated risks were seen 5-9 years after the thyroid cancer diagnosis (SIR = 1.44; 95% CI = 1:14-1.69), and the SIR was close to unity after greater than or equal to 15 years of followup. Previously described elevated risks of subsequent leukemia and breast cancer were not confirmed in this study. Close medical surveillance, thyroid cancer treatment, hereditary factors, and a high frequency of autopsy could all contribute to the elevated risk of a second primary tumor in these patients.