A study of thyroid cancer in patients younger than 20 years showed most tumors to be of the differentiated type. Histological types were pure papillary adenocarcinoma (9.5%), mixed papillary and follicular adenocarcinoma (77%), pure follicular adenocarcinoma (2.7%), and medullary carcinoma (10.8%). Of the patients with differentiated carcinoma, 84% had palpable cervical lymphadenopathy and 11% had pulmonary metastasis on admission. Patients with differentiated carcinoma were followed up for a mean of 15.4 years, and patients with medullary carcinoma for a mean of 7.3 years. No patient died as a result of papillary or follicular carcinoma, but two (25%) died of medullary carcinoma. Permanent hypoparathyroidism was found in 17% of patients after total thyroidectomy.