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<sup>2</sup>, respectively), but in the calcium lactate-treated (CL) group, L2–4 BMD decreased significantly from 0.81 to 0.79 g/cm<sup>2</sup> 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.