Although calcium supplements are widely used in the treatment of osteoporosis their beneficial effect is not conclusively established. The biochemical effects of a calcium supplement (1 g/d) given for between 6 and 36 months were studied in 20 postmenopausal osteoporotic women who had normal calcium absorption. The mean fasting urinary hydroxyproline/creatinine ratio decreased from 0.024 +/- 0.002 to 0.016 +/- 0.001 (P less than 0.005) indicating a significant reduction in bone resorption. The mean plasma alkaline phosphatase fell from 85 +/- 4 U/l to 73 +/- 3 U/l (P less than 0.005), probably representing a secondary reduction in bone formation following the inhibition of bone resorption. There was no significant change in calcium absorption. These results support the concept that calcium supplementation is useful in the treatment of postmenopausal osteoporosis.