Background: Back pain is a major public health problem due to its high frequency, to the resulting activity constraint, and the need for surgery in many cases. Back pain is more frequent in women than men, mainly in postmenopausal women. High prevalence of hypovitaminosis D has been detected in postmenopausal women, and it is associated with decreased bone mass, sarcopenia, vertebral fractures, and inflammation, which can be related to back pain. Methods: The relation between back pain and hypovitaminosis D was evaluated in this study, as well the difference regarding the number of bedridden days, number of days away from work, and daily activities limitation between women with and without hypovitaminosis D. This study reviewed baseline data from an interventional phase III multicenter trial in low bone mass postmenopausal women. The study included demographic data, 25OHD determinations, Newitt/Cummings questionnaire on back pain, and vertebral fracture identified thought X-ray evaluation. Results: The trial included 9354 participants, but only 9305 underwent all the evaluations. The age median was 67 (60 - 85 years old) and age at menopause was 49 (18 - 72 years). Hypovitaminosis D was found in 22.5% of the subjects, 15.3% of them had vertebral fractures, 67.5% with back pain, and 14.8% reduced their daily activities in the previous six months. Subjects with hypovitaminosis D, compared to those without hypovitaminosis D, reported more back pain (69.5 v 66.9%, p: 0.022), more cases of severe back pain (8.5% v 6.8%, p: 0,004), higher limitation in their daily activities (17.2 v 14.0%, p: 0.001), and more fractures (17.4 v 14.6%, p: 0,002); also, they had more trouble to perform daily activities addressed in the Newwit/Cummings questionnaire. Conclusion: Hypovitaminosis D was related to back pain, to its severity, and to difficulty in perform daily activities.