Estrogen therapy was administered to 36 patients with breast cancer: 7 were associated with hypercalcemia. Compared with those with no hypercalcemia, these 7 were somewhat younger, with an average age of 46, and appeared to have more rapidly progressive disease and widespread osteolytic bone metastasis. All failed on estrogen. It is suggested that this type of patient not be continued on estrogen, but instead be considered for adrenalectomy or hypophysectomy or possibly the administration of cytotoxic agents.