Genistein has a preventive role against bone mass loss during menopause. However, experimental data in animal models of osteoporosis suggest an anti-osteoporotic potential for this isoflavone. We performed a post-hoc analysis of a previously published trial investigating the effects of genistein in postmenopausal women with low bone mineral density. The parent study was a randomized, double-blind, placebo-controlled trial involving postmenopausal women with a femoral neck (FN) density < 0.795 g/cm(2). A cohort of the enrolled women was, in fact, identified at the baseline as osteoporotic (n = 121) on the basis of their T-score and analyzed thereafter for the 24 months' treatment with either 1000 mg of calcium and 800 IU vitamin D3 (placebo; n = 59); or calcium, vitamin D3, and Genistein aglycone (54 mg/day; genistein; n = 62). According to the femoral neck T-scores, 31.3% of the genistein and 30.9% of the placebo recipients were osteoporotic at baseline. In the placebo and genistein groups, the 10-year hip fracture probability risk assessed by Fracture Risk Assessment tool (FRAX) was 4.1 +/- 1.9 (SD) and 4.2 +/- 2.1 (SD), respectively. Mean bone mineral density (BMD) at the femoral neck increased from 0.62 g/cm(2) at baseline to 0.68 g/cm(2) at 1 year and 0.70 g/cm(2) at 2 years in genistein recipients, and decreased from 0.61 g/cm(2) at baseline to 0.60 g/cm(2) at 1 year and 0.57 g/cm(2) at 2 years in placebo recipients. At the end of the study only 18 postmenopausal women had osteoporosis in the genistein group with a prevalence of 12%, whereas in the placebo group the number of postmenopausal women with osteoporosis was unchanged, after 24 months. This post-hoc analysis is a proof-of concept study suggesting that genistein may be useful not only in postmenopausal osteopenia but also in osteoporosis. However, this proof-of concept study needs to be confirmed by a large, well designed, and appropriately focused randomized clinical trial in a population at high risk of fractures.