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Introduction. Vitamin D is essential for calcium metabolism as well as for fracture prevention, and a recent review suggested that the optimal serum 25(OH)D lies in the region of 50–80 nmol L−1 (20–32 ng mL−1). A high prevalence of inadequacy has been reported in many studies but the prevalence of inadequacy amongst women with osteoporosis in different regions of the world has not been well characterized. Setting and subjects. A multinational study of 18 countries at various latitudes (range 64N–38S) was conducted in 2004 and 2005 to determine the average levels of serum 25(OH)D and the prevalence of vitamin D inadequacy. A total of 2606 postmenopausal women with osteoporosis (low bone mineral density, history of fragility fracture) seeking routine medical care were enrolled and serum 25(OH)D levels were measured at a single laboratory visit. Results. Mean serum 25(OH)D level was 26.8 ng mL−1 (SE 0.3) and ranged from 7 to 243 ng mL−1. Regional mean values were highest in Latin America (29.6 ng mL−1, SE 0.6) and lowest in the Middle East (20.4 ng mL−1, SE 0.5). Overall, 64% of women had serum levels <30 ng mL−1. Serum parathyroid hormone reached a nadir at serum 25(OH)D levels >35 ng mL−1. In nonequatorial countries, women recruited during the winter months had somewhat lower serum 25(OH)D levels than those recruited during the summer months in some, but not all, countries. Conclusions. Low levels of serum 25(OH)D are common amongst women with osteoporosis. The results underscore the value of assuring vitamin D adequacy in these women. [ABSTRACT FROM AUTHOR]