Among 470 patients with metastatic breast cancer treated with tamoxifen, ten patients (2.3%) developed hypercalcemia. All patients with hypercalcemia had osteolytic or mixed lytic and blastic bone metastases. Hypercalcemia developed after a median period of seven days (range 4-11 days) of tamoxifen administration. Hypercalcemia was treated with conventional measures and serum calcium levels normalized in nine patients, either with a brief interruption of tamoxifen therapy or in spite of continued treatment. Four patients experienced partial remissions with continued tamoxifen therapy. These results indicate that hypercalcemia is a potentially serious complication of tamoxifen therapy but is generally short-lived, and can be controlled with supportive measures, thus allowing continued tamoxifen administration.