Since the association of RET proto-oncogene mutations and medullary thyroid carcinoma in children there has been much discussion regarding timing of surgery. Our study group was formed from a brother and sister (8 and 5) and 3 brothers (9, 13, 16) selected on the basis of a positive family history for thyroid medullary carcinoma. Histological examinations of the thyroidectomy specimens showed that the 8- and 9-year old had microinvasive carcinoma and the remaining three had C-cell hyperplasia. Our recommendation is for prophylactic thyroidectomy for children with RET proto oncogene mutations at an early age, clearly before age 5.