Bone mineral consists of calcium phosphate, and phosphorus is as important as calcium in supporting bone augmentation and maintenance. Although typical adult diets contain abundant phosphorus, 10% to 15% of older women have intakes of less than 70% of the recommended daily allowance. When these women take high-dose calcium supplements that consist of the carbonate or citrate salts, all their food phosphorus may be bound and hence unavailable for absorption. Current-generation anabolic agents for treating osteoporosis require positive phosphorus balances of up to 90 mg/d. Attention to the nutritional adequacy of the diets of such patients is essential if they are to realize the full potential of such therapies. A calcium phosphate supplement may be preferable to the usual carbonate or citrate salts because its phosphate serves to spare food phosphorus.