Background: In vitro studies have shown that genistein inhibits the CYP240 enzyme, which is involved in the degradation of 1,25-dihydroxycholecalciferol and its precursor 25-hydroxycholecalciferol, and increases their plasma levels. However, no clinical studies have primarily assessed the synergistic effect of isoflavones on vitamin D levels. The aim of this study was to evaluate the possible additive effect of genistein supplementation on vitamin D levels, calcium metabolism and bone remodeling markers in healthy postmenopausal women during the spring-summer months. Patients and methods: We made a prospective, double-blind study with 150 healthy postmenopausal women that were randomized to three groups. One received placebo, another received calcium (1000 mg/day) and vitamin D (cholecalciferol, 800 U/day) and the third received calcium (1000 mg/day), vitamin D (cholecalciferol, 800 U/day) and genistein (90 mg/day). The study period was from May to September (spring-summer). Vitamin D, PTH, CTX and P1NP were determined by electrochemiluminescence at baseline and after 12 weeks. Results: Vitamin D levels increased in all groups: placebo (23±9 ng/ml vs. 29±10 ng/ml, p<0.05), calcium+vitamin D (26±10 ng/ml vs. 33±8 ng/ml, p<0.05) and calcium+vitamin D+genistein (24±9 ng/ml vs. 31±8 ng/l, p<0.05) without between-group differences. At study end, the percentage of women with vitamin D <20 ng/ml (11%) and <30 ng/ml (39%) had fallen without between-group differences. The effects on calcium metabolism and bone remodeling markers were similar between groups: rises in vitamin D were significantly linked to reductions in PTH, CTX and P1NP. Conclusion: Adding genistein to supplementation with calcium and vitamin D provided not additional changes in vitamin D levels, calcium metabolism or bone remodeling markers in healthy Spanish postmenopausal women during the spring-summer months.