Recent studies have shown that low serum 25-hydroxyvitamin D (25[OH]D) level is a risk factor for preeclampsia. The clinical significance of in vitro findings that vitamin D regulates vascular endothelial growth factor production is unclear. We sought to determine whether there is an association between midgestation serum 25(OH)D levels and angiogenic factor activity and to compare their predictive value for the development of severe preeclampsia. We conducted a nested case-control study of women with severe preeclampsia (n=41) versus women with uncomplicated term birth (n=123) who had second trimester genetic screening (15-20 weeks). Using banked frozen serum, we measured levels of 25(OH)D, vascular endothelial growth factor, soluble fms-like tyrosine kinase 1, and placental growth factor and compared their correlations and predictive values. We found no correlation between serum 25(OH)D and angiogenic factors levels. 25(OH)D alone was comparable to vascular endothelial growth factor and soluble fms-like tyrosine kinase 1/placental growth factor ratio as a predictive marker for severe preeclampsia. A composite of both 25(OH)D level and soluble fms-like tyrosine kinase 1/placental growth factor ratio was more predictive than either alone (area under curve: 0.83 versus 0.74 and 0.67, respectively). In conclusion, combining midpregnancy 25(OH)D level with soluble fms-like tyrosine kinase 1/placental growth factor ratio provides a better prediction for the development of severe preeclampsia. © 2011 American Heart Association, Inc.