Very few childhood cancer survivor studies have been devoted to thyroid adenomas. We assessed the role of chemotherapy and the radiation dose to the thyroid in the risk of thyroid adenoma after childhood cancer. A cohort of 3254 2-year survivors of a solid childhood cancer treated in 5 French centers before 1986 was established. The dose received by the isthmus and the 2 lobes of the thyroid gland during each course of radiation therapy was estimated after reconstruction of the actual radiation therapy conditions in which each child was treated as well as the dose received at other anatomical sites of interest. After a median follow-up of 25 years, 71 patients had developed a thyroid adenoma. The risk strongly increased with the radiation dose to the thyroid up to a few Gray, plateaued, and declined for high doses. Chemotherapy slightly increased the risk when administered alone but also lowered the slope of the dose-response curve for the radiation dose to the thyroid. Overall, for doses up to a few Gray, the excess relative risk of thyroid adenoma per Gray was 2.8 (90% CI: 1.2-6.9), but it was 5.5 (90% CI: 1.9-25.9) in patients who had not received chemotherapy or who had received only 1 drug, and 1.1 (90% CI: 0.4-3.4) in the children who had received more than 1 drug (P=.06, for the difference). The excess relative risk per Gray was also higher for younger children at the time of radiation therapy than for their older counterparts and was higher before attaining 40 years of age than subsequently. The overall pattern of thyroid adenoma after radiation therapy for a childhood cancer appears to be similar to that observed for thyroid carcinoma.