BACKGROUND An imbalance of angiogenesis-associated factors may predispose to preeclampsia. Here, we determined the ratio of serum concentration of soluble fms-like tyrosine kinase 1 (sFlt-1), a natural inhibitor of pro-angiogenic vascular endothelial growth factor (VEGF) relative to angiopoietin-2 (Ang-2), a natural antagonist of angiopoietin-1 (Ang-1) involved in promoting angiogenesis in the presence of VEGF, in women with preeclampsia. METHODS The levels of serum sFlt-1 and Ang-2 were measured by enzyme-linked immunosorbent assay. RESULTS Significant decrease of serum Ang-2 and the increase of sFlt-1 were observed in women with preeclampsia as compared to healthy pregnant women. The serum sFlt-1/Ang-2 ratio was strikingly increased in preeclamptic women in contrast to healthy pregnant women exhibiting lower value similar to non-pregnant women. The serum sFlt-1 concentrations tended to positively correlate with mean blood pressure (BP) in preeclamptic women, but not in healthy pregnant women. A cut-off value >0.25 in the serum sFlt-1/Ang-2 ratio showed 87.1% sensitivity and 82.8% specificity in differentiating preeclamptic women from healthy pregnant women. CONCLUSION The serum sFlt-1/Ang-2 ratio is significantly elevated in preeclamptic women as compared to healthy pregnant women. Remarkable difference of sFlt-1/Ang-2 ratio between these two groups with excellent specificity and sensitivity suggests the clinical usefulness of the serum sFlt-1/Ang-2 ratio in diagnosing and potentially predicting the onset of preeclampsia.