Because genetic predisposition probably plays an important role in the aetiology of most of childhood cancers, studies of second primaries occurring after these cancers may be particularly informative about possible common genetic mechanisms in both of these cancers. We have studied the incidence of thyroid tumours occurring after cancer in childhood in a cohort of 592 children treated before 1970. Among these children, six later developed a thyroid carcinoma, and 18 developed a thyroid adenoma. Radiation doses received to the thyroid by each of the irradiated children have been estimated using individual radiotherapeutic technical records. Thyroid carcinomas and thyroid adenomas were five times more frequent after irradiation for neuroblastoma than after irradiation for any other first cancer. This ratio did not depend on sex, nor on time elapsed since irradiation, nor on dose of radiation received for the thyroid gland. This result suggests that there is a common mechanism for the occurrence of neuroblastoma and of differentiated thyroid tumour.