The purpose of this communication is to record the occurrence of hyperplasia of the thyroid in newborn twins following the administration of thiouracil and iodine to the mother during gestation. To our knowledge no other case of apparently induced hyperplasia of the thyroid in humans has been reported. Experimentally hyperplasia of the thyroid has been produced in puppies by thyroidectomy upon the mothers. In regions where endemic goiter prevails, the occurrence of spontaneous congenital enlargement of the thyroid is not rare. REPORT OF CASE History.— The first of normally delivered twin girls was stillborn. The mother, aged 36, who had previously borne six healthy children, was receiving treatment for thyrotoxicosis. For five months she had received strong iodine (Lugol's) solution, 10 drops three times daily, and propylthiouracil, 100 mg. four times daily, with amelioration of her symptoms and reduction of the basal metabolic rate from +36 to +11%. External Examination.—