Many observers have reported the postnatal development of various disease entities in either one or both of fraternal and identical twins. There have been many hypotheses advanced as to the operating causes producing these changes. A perusal of the literature reveals that marked primary hyperplasia of the thyroid in one of still-born twins, probably identical, has not been reported. Because of the relatively early stage of fetal development a clearer evaluation of the inciting factors involved can be obtained in this instance. REPORT OF CASE History. —C. S., a woman, aged 34, admitted to the Lahey Clinic September 29, complained of a swelling of the neck of two years' duration. The patient said that there were no symptoms of toxicity except for a loss of from 30 to 35 pounds (14 to 16 Kg.) over a period of three years. The patient, a quadripara, at this time