Ipriflavone (IP) (7-isopropoxyisoflavone), a synthetic isoflavone derivative, is active in both inhibiting bone resorption and enhancing osteoblast function. This property suggested its clinical use in the treatment of involutional osteoporosis, and in the prevention of postmenopausal bone mass loss. Forty postmenopausal women with low bone mineral content were enrolled and randomly treated for 12 months with IP 600 mg/day or placebo (PL), according to a double-blind, parallel group design. All patients wee also given an oral calcium supplementation (1 g/day). Bone mineral density (BMD) was measured at the spine (L2-L4) by dual-energy X-ray absorptiometry and at the distal radius by single-photon absorptiometry. Bone metabolism markers (serum calcium, phosphate, osteocalcin, and alkaline phosphatase, and urinary calcium, phosphate, and hydroxyproline) were assessed at the same times. After 12 months, a reduction of BMD was evidenced in the PL-treated group, at both the spine (-2.2%, P < 0.01 vs baseline) and the forearm (-1.2%). In the IP-treated group, an increase of BMD was obtained (+1.2%, P < 0.01 vs placebo, at the spine; +3%, not significant, at the forearm). Bone markers were in the normal range for postmenopausal women; no statistically significant modifications were observed during the treatment period. Three patients were withdrawn from the treatment in the IP-treated group, and two in the PL-treated group for gastrointestinal disturbances. In the other women, the tolerance of the drug was good and the compliance with the oral treatment was excellent.