Gallium nitrate was recently found to be effective treatment for resistant cancer-related hypercalcemia. In vitro and in vivo experiments have suggested that the drug directly inhibits calcium resorption from bone; however, the overall effects of gallium nitrate on calcium balance were unknown. We have completed metabolic balance studies in four patients who received this drug by prolonged infusion. All patients were in positive calcium balance while receiving the drug. Each patient also showed a substantial decrease in urinary calcium excretion. Serum phosphorus decreased in all four patients. There was no change in phosphorus, sodium, chloride, or magnesium balance or in creatinine clearance. We conclude that prolonged infusions of gallium nitrate reduce urinary calcium excretion and that the hypocalcemic effect of this drug is primarily due to inhibition of calcium resorption from bone. Thus, the drug may prove useful in reducing accelerated bone resorption in patients with bone metastases or chronic cancer-related hypercalcemia.