The efficacy of ipriflavone was investigated in a 1-year double-blind, placebo-controlled, parallel group clinical trial. Ninety-one postmenopausal women completed the study, 41 received ipriflavone and 50 placebo treatment. After six months the bone mineral density of the L2-L4 vertebral region increased in the ipriflavone-treated group (0.015 g/cm2), whereas it decreased in the placebo-treated group. The differences between the treatment groups were statistically significant. Our results support the efficacy of ipriflavone in the treatment of postmenopausal osteoporosis. Since the positive effect was more pronounced after 6 months, the possibility of an intermittent ipriflavone treatment might be taken into consideration in the future.