['It remains unclear whether vitamin D sufficiency optimizes response to bisphosphonate (BP) treatment in postmenopausal osteoporosis. We evaluated the role and possible mechanisms of vitamin D in adequate response to standard BP treatment for postmenopausal osteoporosis.', 'We included 120 postmenopausal osteoporotic women (aged 68 ± 8 years) receiving BP (alendronate or risedronate) at their annual follow-up, performing complete anamnesis, including treatment adherence, use of vitamin D supplements, and previous falls and fractures during the last year. We analyzed the evolution of bone mineral density (BMD) during this period and serum PTH and 25 hydroxyvitamin D (25(OH)D) and urinary NTx levels. Patients were classified as inadequate responders to antiosteoporotic treatment based on BMD loss>2% and/or the presence of fragility fractures during the last year.', 'Thirty percent of patients showed inadequate response to BP treatment, with significantly lower levels of 25(OH)D (22.4 ± 1.3 vs. 26.6 ± 0.3 ng/ml, p=0.01), a higher frequency of 25(OH)D levels<30 ng/ml (91% vs. 69%, p=0.019) and higher urinary NTx values (42.2 ± 3.9 vs. 30.9 ± 2.3 nM/mM, p=0.01). Patients with 25(OH)D>30 ng/ml had a greater significant increase in lumbar BMD than women with values <30 ng/ml (3.6% vs. 0.8%, p<0.05). The probability of inadequate response was 4-fold higher in patients with 25(OH)D<30 (OR, 4.42; 95% CI, 1.22-15.97, p=0.02).'] Do 25 hydroxyvitamin D serum levels influence adequate response to bisphosphonate treatment in postmenopausal osteoporosis?