In vitro studies have shown that ipriflavone affects both bone formation and bone resorption, but the effect in early-stage postmenopausal women with low bone mass and a high turnover of bone metabolism is unknown. In this prospective study, we randomly assigned 60 patients with postmenopausal osteopenia or osteoporosis to receive either 600 mg/day of ipriflavone or 0.8 g/day calcium lactate, and compared the effects on bone mineral density (BMD) from the 2nd to 4th lumbar vertebrae (L2-4) and bone metabolic markers before and after one year of treatment. In the iprifravone-treated (IP) group, L2-4 BMD was similar before and after treatment (0.78 and 0.77 g/cm(2), respectively), but in the calcium lactate-treated (CL) group, L2-4 BMD decreased significantly from 0.81 to 0.79 g/cm(2) after 1 year of treatment (p < 0.0001). Furthermore, the rate of the decrease in L2-4 BMD was significantly greater in the CL group than in the IP group (p < 0.01). The median deoxypyridinoline (Dpd) level was significantly lower after 1 year of treatment (5.8 mmol/mmol creatinine [Cr]) than the baseline value (10.2 mmol/mmol Cr) in the IP, but not in the CL group, suggesting that IP treatment suppresses bone resorption.