ESTROGEN has proven to be of great value in the treatment of metastatic mammary carcinoma. Increased well-being, symptomatic relief, objective evidence of tumor regression, and even prolonged survival have been obtained in the face of advanced local and generalized carcinomatosis. This agent is particularly effective after the tenth postmenopausal year. Estrogen has been more effective than androgen. The toxic side effects of estrogen therapy are not inconsequential. Gastrointestinal upset, pigmentation, urinary frequency, vaginal bleeding, saltretention edema, and hypercalcemia have been observed. Hypercalcemia, in particular, may be quite dangerous. In 1949, Hermann and others reported 4 cases of lethal hypercalcemia following the administration of androgen and estrogen. Since that time admonitions regarding this possible side effect have appeared repeatedly in the literature dealing with sex steroid therapy of carcinomatosis. Two recent hypercalcémie complications of estrogen therapy at the Breast Service of the Francis Delafield Hospital prompted a review of our experience