The relationship between ovarian secretion and the growth and development of the breasts has been observed clinically for many years. With increasing knowledge of ovarian function, especially along hormonal lines, evidence is being accumulated to suggest the dependency of mammary activity on ovarian and anterior pituitary hormones. The former cause development of the breasts at puberty and their rhythmic changes associated with the menstrual cycle. The latter, when pregnancy intervenes with its preparation of the breasts for lactation, induce the secretion of milk. Because the ovarian hormones, especially the estrogenic principle, are growth-promoting factors in normal mammary development and because cancer is essentially an unrestricted growth, it is not illogical to deduce that in pathologic conditions of the breast, such as carcinoma, these hormones may stimulate malignant as well as mammary growth. The hypothesis advanced in this paper, therefore, is that in patients with cancer of the breast, whether it