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Introduction: The role of vitamin D on bone microarchitecture and fragility is not clear. Objective: To investigate whether vitamin D deficiency (25(OH)D <20 ng/mL) increases cortical bone loss and the severity of fractures. Design: Cross-sectional study of 287 elderly women with at least one prevalent low-impact fracture. Methods: Biochemistry, X-rays to identify vertebral fractures (VFs) and to confirm non-vertebral fractures (NonVFs), and high-resolution peripheral quantitative computed tomography (HR-pQCT) to evaluate bone microstructure. Results: Serum 25(OH)D levels were associated with body mass index (BMI: r = -0.161, P = 0.006), PTH (r = -0.165; P = 0.005), CTX (r =-0.119; P = 0.043) and vBMD at cortical bone (Dcomp: r = 0.132; P = 0.033) and entire bone (D100: r=0.162 P = 0.009) at the distal radius, but not at the tibia. Age and PTH levels were potential confounding variables, but in the multiple linear regressions only BMI (95% CI: 0.11-4.16; P< 0.01), 25(OH)D (95% CI: -0.007 to 1.70; P = 0.05) and CTX (95% CI: -149.04 to 21.80; P<0.01) predicted Dcomp, while BMI (95% CI: 1.13-4.18; P<0.01) and 25(OH)D (95% CI: 0.24-1.52; P < 0.01) predicted D100. NonVFs predominated in patients with 25(OH)D <20 ng/mL (P = 0.013). Logistic regression analysis showed a decrease in the likelihood of presenting grade 2-3 VFs/NonVFs for every increase in 25(OH)D (OR = 0.962, 95% CI: 0.940-0.984; P = 0.001), BMI (OR = 0.932, 95% CI: 0.885-0.981; P = 0.007) and D100 at radius (OR = 0.994, 95% CI: 0.990-0.998; P = 0.005). Conclusion: In elderly patients with prevalent fractures, vitamin D deficiency was associated with cortical bone loss and severity of fractures. [ABSTRACT FROM AUTHOR]