SummaryFour cases of metastatic mammary cancer are reported. These patients had a persistent elevation of the serum calcium level and a marked increase of urinary calcium. In two cases in whom postmortem examination was obtained, nephrocalcinosis was present.The hypercalcemia was accompanied by renal insufficiency and blocd nitrogen retention.3o The hypercalcemia was the result of bone destruction by the tumor and in certain cases by the administration of sexual hormones.4. The dangers from hypercalcemia make it advisable to study the effect of sexual hormones on the calcium metabolism of every patient with metastatic mammary cancer in whom hormonotherapy is contemplated.