Thyroid neoplasms occurred in two patients 14 and 18 years after treatment with radiation for medulloblastoma. One patient has a papillary cancer and the other patient had multiple adenomas and a Hurthle cell adenoma. In addition, the latter case had foci of hyperplasia plus a microfollicular transformation, which has not been described in association with prior thyroid irradiation. The radiation doses delivered to the thyroid through posterior cervical spinal ports (2000--3000 roentgens) fall within the upper range of radiation dosage associated with induction of neoplastic changes within the thyroid.