Low levels of serum 25 Hydroxyvitamin D [25(OH)D] have been linked to greater fracture risk in older women. This study aimed to determine whether higher 25(OH)D is associated with slower loss of bone mineral density (BMD) and lower fracture risk during the menopausal transition. This was a prospective cohort study at five clinical centers in the United States. Mean age was 48.5 ± 2.7 years. The fracture analysis included 124 women with an incident traumatic fracture, 88 with incident nontraumatic fracture, and 1532 women without incident fractures; average followup was 9.5 years. BMD analysis included 922 women with a documented final menstrual period. Serum 25(OH)D was measured by liquid chromatography tandem mass spectrometry at the third annual clinic visit. BMD was measured and incident fractures ascertained at each annual visit. The mean 25(OH)D was 21.8 ng/mL; seven-hundred two (43%) of the women had 25(OH)D values <20 ng/mL. There was no significant association between 25(OH)D and traumatic fractures. In multivariate adjusted hazards models, the hazard ratio (HR) for nontraumatic fractures (95% confidence interval [CI]) was 0.72 (0.54-0.96) for each 10-ng/mL increase in 25(OH)D. Comparing women whose 25(OH)D was ?20 vs <20 ng/mL, the HR (95% CI) for fracture was 0.54 (0.32-0.89). Changes in lumbar spine and femoral neck bone mineral density across menopause were not significantly associated with serum 25(OH)D level. Serum 25(OH)D levels are inversely associated with nontraumatic fracture in mid-life women. Vitamin D supplementation is warranted in midlife women with 25(OH)D levels <20 ng/mL.