Background: The thyroid gland is known to be sensitive to the carcinogenic effect of ionizing radiation, especially in children. The role of potential modifiers of the risk and latency period effects needs further investigation. We examined the effect of low doses of ionizing radiation (4.5-49.5 cGy) on the risk of developing thyroid cancer after long latent periods of up to 54 yr after childhood exposure. Methods: The study population included 10,834 individuals irradiated against tinea capitis in the 1950s and two matched nonirradiated groups (general population and siblings) for comparison. Cancer statistics and vital status data were obtained from national registries, updated to December 2002. Excess relative and absolute risks [excess relative risk per gray (ERR/Gy), excess absolute risk (EAR)] were estimated using Poisson regression for survival analysis. Results: Within the study period, 159 cases of thyroid cancer were diagnosed. Total ERR/Gy and excess absolute risk per gray per 104 person-years for developing thyroid cancer reached 20.2 (95% confidence interval 11.8-32.3) and 9.9 (95% confidence interval 5.7-14.7), respectively. The risk was positively associated with dose and negatively associated with age at exposure. ERR/Gy was significantly elevated 10-19 yr after exposure, peaking at 20-30 yr, and decreasing dramatically (although still significantly elevated) 40 yr after exposure. Conclusions: Our findings agree with patterns of risk modification seen in most studies of radiation-induced thyroid cancer, although risk per unit dose seems higher. Our data show that 40 yr after irradiation, ERR decreases dramatically, although remaining significantly elevated. The hypothesis of different genetic susceptibility of the Jewish population deserves further exploration.