• Responses to the administration of androgens and estrogens were studied in four women in whom there was cancer of the breast with bony metastases. On the basis of the idea that active growth of tumor in bone results in liberation of calcium secondary to increased osteolysis, the responses of the tumor to these hormones were measured by changes in the level of serum calcium and in calcium excretion. Hypercalcemia and hypercalcuria developed after administration of both hormones, and urinary calcium measurements proved to be an excellent guide to impending hypercalcemia. It is concluded that either estrogen or androgen may stimulate the growth of mammary cancer and that a possible mechanism underlying this observation is the conversion of androgen to estrogen in the human.