BACKGROUND: The relationship between serum 25-hydroxyvitamin D [25(OH) vitamin D] concentration and hip fractures is unclear. OBJECTIVE: To see whether low serum 25(OH) vitamin D concentrations are associated with hip fractures in community-dwelling women. DESIGN: Nested case-control study. SETTING: 40 clinical centers in the United States. PARTICIPANTS: 400 case-patients with incident hip fracture and 400 control participants matched on the basis of age, race or ethnicity, and date of blood draw. Both groups were selected from 39 795 postmenopausal women who were not using estrogens or other bone-active therapies and who had not had a previous hip fracture. MEASUREMENTS: Serum 25(OH) vitamin D was measured and patients were followed for a median of 7.1 years (range, 0.7 to 9.3 years) to assess fractures. RESULTS: Mean serum 25(OH) vitamin D concentrations were lower in case-patients than in control participants (55.95 nmol/L [SD, 20.28] vs. 59.60 nmol/L [SD, 18.05]; P = 0.007), and lower serum 25(OH) vitamin D concentrations increased hip fracture risk (adjusted odds ratio for each 25-nmol/L decrease, 1.33 [95% CI, 1.06 to 1.68]). Women with the lowest 25(OH) vitamin D concentrations (< or =47.5 nmol/L) had a higher fracture risk than did those with the highest concentrations (> or =70.7 nmol/L) (adjusted odds ratio, 1.71 [CI, 1.05 to 2.79]), and the risk increased statistically significantly across quartiles of serum 25(OH) vitamin D concentration (P for trend = 0.016). This association was independent of number of falls, physical function, frailty, renal function, and sex-steroid hormone levels and seemed to be partially mediated by bone resorption. LIMITATIONS: Few case-patients were nonwhite women. Bone mineral density and parathyroid hormone levels were not accounted for in the analysis. CONCLUSION: Low serum 25(OH) vitamin D concentrations are associated with a higher risk for hip fracture.