Simple mastectomy is an operation frequently attempted but seldom accomplished. Bold as this declaration may seem, it is true. Theoretically the term "mastectomy" indicates complete excision of the mammary gland and does not apply to incomplete removal, partial resection or subtotal ablation. In practice, however, the vast majority of so-called "mastectomies" accomplish nothing more than incomplete extirpation of the main mass of mammary tissue. While the surgeon aims to remove all of the mammary gland, he invariably leaves some of it behind to undergo either normal involution or pathologic change. During the past three years, careful anatomic and histologic studies have been made of all breasts sent to the pathologic laboratories of several large hospitals. The mastectomies had been done by well trained surgeons, many of whom had had considerable experience in surgical treatment of the breast. In practically every instance no difficulty was encountered in demonstrating that the breast