Although calcium supplements are widely used to reduce bone resorption in osteoporosis, their beneficial effect is not conclusively established. We studied the effect of a calcium supplement (1 g/day for 8 days) in 14 postmenopausal osteoporotic women. The fasting urinary hydroxyproline/creatinine ratio decreased from 0.022 +/- 0.001 to 0.017 +/- 0.001 (p less than 0.005) indicating a significant reduction in bone resorption. Both the tubular maximum for phosphate reabsorption (1.12 +/- 0.06 to 1.34 +/- 0.07, p less than 0.005) and plasma phosphate concentration (1.04 +/- 0.04 to 1.14 +/- 0.04, p less than 0.01) increased, consistent with suppression of parathyroid hormone activity. These results support the concept that calcium supplementation is useful in the treatment of postmenopausal osteoporosis.