Although thyroid cancer tends to metastasize early in children, it is generally associated with a good prognosis. In this study, the expression of p53, mutations of which are found in many cancers, including anaplastic thyroid cancer, was examined to determine the relationship between cell proliferation and the clinical course of thyroid cancer. The clinicopathological findings and clinical courses of 15 children who underwent surgery before the age of 18 years at our hospital between 1972 and 1992 were examined, and the expression of p53 was studied using immunohistochemical techniques and an RNase protection assay. Postoperative follow-up ranged from 1 to 20 years, with a median of 12 years. No abnormal expression of p53 was detected in the thyroid cancer of any of the children tested, and none of them have died. The findings of this study therefore strongly suggest that p53 may play a role in the regulation of cell proliferation, and in this capacity slow the growth of and be related to the prognosis of differentiated thyroid cancer in children.