[en] To provide quantitative data on the risk of thyroid cancer following ¹³¹I exposure, 34104 patients administered ¹³¹I for diagnostic purposes were followed for up to 40 years. Mean thyroid dose was estimated as 1.1 Gy, and 67 thyroid cancers occurred in contrast to 49.7 expected [standardized incidence ratio (SIR)=1.35; 95% confidence interval (CI) 1.05-1.71]. Excess cancers were apparent only among patients referred because of a suspected thyroid tumor and no increased risk was seen among those referred for other reasons. Further, risk was not related to radiation dose to the thyroid gland, time since exposure, or age at exposure. The slight excess of thyroid cancer, then appeared due to the underlying thyroid condition and not radiation exposure. Among those under age 20 years when ¹³¹I was administered, a small excess risk (3 cancers vs 1.8 expected) was about 2-10 times lower than that predicted from A-bomb data. These data suggest that protraction of dose may result in a lower risk than acute x-ray exposure of the same total dose