The mean calcium requirement of normal adults has been estimated from 212 calcium balances on 84 normal subjects and found to be 578 mg. The calcium allowance required to ensure that 95% of normal adults are in calcium balance is about 900 mg. Calcium requirement is relatively higher than that of phosphorus or magnesium because plasma calcium--and therefore urine calcium--is maintained at the expense of the skeleton in the presence of calcium deficiency, whereas plasma phosphate and magnesium--and therefore urine phosphate and magnesium--fall on phosphorus and magnesium deficient diets. Calcium requirement appears to rise at the menopause, and postmenopausal bone loss can be reduced by calcium supplements. In postmenopausal osteoporosis, the severe negative calcium balance can be corrected by giving calcium or calcium and vitamin D, but vitamin D alone is not beneficial. Similarly, bone loss, height loss and further vertebral compression can be reduced by calcium or calcium and vitamin D but not by vitamin D alone.