A retrospective analysis was conducted on 36 patients who underwent surgery for childhood or juvenile thyroid cancers and who were regularly followed up during the course of 30 years. The biological properties and late prognosis of these patients were assessed, and the clinical and morphological characteristics of the tumors were examined. The distribution of the DNA content in the tumor cells was compared with that in adult tumor cells, and the results of cytogenetic tests performed on mothers operated on for thyroid tumors and their children are also discussed.