Extensive endocrine and histological studies were performed in 3 hyperthyroid patients with coexistent gynecomastia. In each case, the onset of mastodynia was during the hyperthyroidism, and the gynecomastia receded with return to the euthyroid state. Sequential breast biopsies were performed before and after control of hyperthyroidism, and, in one patient, sequential liver and testicular biopsies were done. The initial appearance of the breast and the subsequent regression of the gynecomastia were studied histologically and histochemically. Initially, the ducts were elongated and tortuous, the epithelium was hyperplastic and papillary, and the periductal stroma was loose and myxomatous. After the hyperthyroidism was controlled, the epithelium receded in height, the ducts were less branched, and the stroma became dense and collagenous. The large amounts of acid mucopolysaccharides in the periductal stroma diminished with regression of the gynecomastia. The intracellular glycogen in the epithelium persisted. The hormonal studies did not identify the humoral mechanism causing the gynecomastia in these patients. The literature of gynecomastia associated with hyperthyroidism was reviewed. It is suggested that gynecomastia may occur more commonly in patients with hyperthyroidism than heretofore assumed.