30 patients with disorders of calcium metabolism were treated with dichloromethylene diphosphonate (C1(2)MDP, or clodronate disodium), an inhibitor of bone resorption. 13 patients with Paget's disease of bone were given C1(2)MDP by mouth (1.6 g/day). Serum-alkaline-phosphatase and urinary hydroxyproline fell to normal or near-normal within 3-7 months, and there was a clinical improvement in all but 1 patient. C1(2)MDP (0.8-3.2 g/day) also reduced plasma-calcium and urinary calcium in 17 patients with hypercalcaemia due to primary hyperparathyroidism or secondary to malignant disease. C1(2)MDP seems to be an effective oral drug for inhibiting excessive bone resorption in man.