A prospective, cohort study of the association between blood levels of 1,25-dihydroxyvitamin D and incident hypertension among US women. Vitamin D deficiency has been associated with hypertension in observational studies. However, findings from clinical trials of vitamin D supplementation for prevention of hypertension have been inconsistent. Therefore, we evaluated the association between serum levels of 1,25-dihydroxyvitamin D (1,25-D) and risk of hypertension among apparently healthy women. A total of 733 women (age: 46.8 ± 9.0 y) who were free of hypertension at baseline were prospectively followed from 2003 through 2007. We evaluated the association between blood levels of 1,25-D and risk of incident hypertension using logistic regression models. We also examined the association between blood levels of 1,25-D and incident hypertension after stratification by body mass index (BMI), race, and baseline blood pressure status. Among all women, each 1-SD higher blood level of 1,25-D was associated with a decreased odds of incident hypertension (odds ratio (OR): 0.68; 95% confidence interval (CI): 0.48-0.96; P = 0.03). Further, among overweight or obese women (n = 430), each 1-SD higher blood level of 1,25-D was associated with a decreased odds of hypertension (OR: 0.49; 95% CI: 0.29-0.83; P = 0.007). In conclusion, our findings suggest a potential role for vitamin D in the prevention of hypertension, especially among overweight or obese women. Future studies are warranted to replicate our results.