Dichloromethylene diphosphonate (Cl2MDP), an inhibitor of osteoclast-mediated bone resorption, lowers serum calcium in hypercalcemia associated with malignancies and with primary hyperparathyroidism. We have evaluated the effectiveness of Cl2MDP in three patients who had multiple osteolytic bone metastases due to breast cancer and coincident primary hyperparathyroidism and who refused neck exploration. Cl2MDP was added to the tamoxifen treatment and was given orally at a dose of 1600 mg/day for 12 months. All patients had a reduction in serum calcium level which was accompanied by a decline in the fasting urinary calcium and hydroxyproline excretion. Administration of Cl2MDP was not associated with any changes in parathyroid hormone levels. New bone metastases were observed neither during the treatment nor in the follow-up period. No side effects were observed.