The purpose of this study was to determine whether the levels of soluble fms-like tyrosine kinase-1 (sFlt-1) and placenta growth factor (PlGF) are altered during the second trimester in the plasma of women who subsequently develop preeclampsia. We performed a case-control study to compare the levels of sFlt-1 and PlGF in the preeclamptic (n=46) and normal pregnant women (n=100). The maternal plasma levels of sFlt-1 and PlGF were measured by enzyme-linked immunosorbent assay. The sFlt-1 levels were significantly higher in the preeclamptic women than in normal controls (p<0.001), while the PlGF levels were significantly lower (p<0.001). In normal controls, sFlt-1 levels were positively correlated (r=0.27, p=0.008), whereas, in the preeclamptic women, those were negatively correlated with the PlGF levels (r=-0.423, p=0.005). Furthermore, the log[sFlt-1/PlGF] ratio was significantly higher in the preeclamptic women than in normal controls (p<0.001). The receiver operating characteristic curve revealed a specificity of 78% with a diagnostic sensitivity of 80.4%; the optimal cut-off value of the log[sFlt-1/PlGF] ratio was 1.4 (95% CI 0.756-0.910, p<0.001). Preeclampsia showed a strong association with increased levels of sFlt-1 and decreased levels of PlGF in the second trimester maternal plasma. Accordingly, the sFlt-1/PlGF ratio may provide early prediction of subsequent development of preeclampsia.