Objective: To examine the effect of phosphate supplements on calcium homeostasis and bone turnover in young men. Design: Study 1 was a randomised, controlled, cross-over trial of 1000 mg elemental phosphate given for one week, with a standard diet of 800 mgad each of calcium and phosphorus. Study 2 was an escalating dose study of 0, 1000, 1500 and 2000 mgad elemental phosphate, each given for one week, with a standard diet of 1000 mgad each of calcium and phosphate. Setting: Northern General Hospital, Shef®eld. Subjects: Study 1, 10 healthy men ages 19±32 y. Study 2, 12 healthy men ages 19±38 y. Interventions: Sodium acid phosphate tablets each containing 500 mg elemental phosphorus (16.1 mmolad), given with meals. Results: Study 1, administration of 32.2 mmolad phosphorus resulted in a mean increase of 13.2 mmolad urinary phosphorus, a mean decrease of 1.1 mmolad urinary calcium, and a mean increase of 7 ngamL in serum intact PTH. There were no changes in serum phosphate, osteocalcin or urinary N-telopeptide excretion. Study 2, administration of 64.4 mmolad phosphorus resulted in a mean increase of 27.2 mmolad urinary phosphorus, a mean decrease of 2.4 mmolad urinary calcium, with no change in serum phosphate, PTH or urinary deoxypyridinoline. Conclusions: Phosphate supplementation of the diet does not affect bone turnover in young men.