The effect of phosphorus (inorganic phosphate) supplementation was studied in seven postmenopausal women with osteoporosis. Prior to supplementation, all chemical parameters studied in serum and urine were normal. Bone density was below the fifth percentile for age in all but one patient, and the percentage of bone surface involved in resorption was higher than normal. During administration of the phosphorus supplement, fasting serum concentrations of calcium and immunoreactive parathyroid hormone showed no significant changes, while serum phosphorus, urinary calcium, and tubular reabsorption of phosphorus decreased. In four patients studied by balance techniques, calcium balance became positive or less negative. Bone-forming surface decreased and bone-resorbing surface increased in all patients. Bone-resorbing surface was highly correlated with total phosphorus intake. Density of the distal radius changed variably, while density of the midradius increased slightly in all patients.