Preeclampsia is the leading cause of morbidity and mortality in pregnancy. Although the etiology of preeclampsia is still unclear, it is believed to involve rejection of the fetus, possibly due to an imbalance between regulatory (Treg) and effector T cells. To test this, we compared the frequencies of circulating CD4(+) T cells expressing Foxp3, IFN-gamma, IL-10, or IL-17 at the end of the third trimester of healthy and preeclamptic pregnancies. The size of the Treg cell compartment, defined by the frequency of CD4(+)CD25(high), CD4(+)CD127(low)CD25(+), and CD4(+)Foxp3(+) cells was significantly higher in normal compared with preeclamptic pregnancies. CD4(+)CD25(high) and CD4(+)CD127(low)CD25(+) populations in preeclampsia were not significantly different from those in nonpregnant controls, whereas CD4(+)Foxp3(+) cells numbersre slightly lower in preeclampsia. The suppressive activity of ex vivo-sorted CD4(+)CD127(low)CD25(+) Treg cells was not significantly different between the three study groups. The percentage of CD4(+)IL-17-producing T cells decreased significantly in healthy compared with preeclamptic pregnancies and nonpregnant controls, whereas CD4(+)IL-10- and CD4(+)IFN-gamma-producing cells remained unchanged. Consequently, the ratio of Foxp3(+) Treg to IL-17-expressing CD4(+) T cells was significantly increased in healthy but not in preeclamptic pregnancies. Thus, preeclampsia is associated with the absence of normal systemic skewing away from IL-17 production toward Foxp3(+) expression. Finally, preeclamptic women had significantly higher levels of soluble endoglin, an inhibitor of TGF-beta receptor signaling, which may bias toward IL-17 production. These results suggest that homeostasis between regulatory and proinflammatory CD4(+) T cells might be pivotal for the semiallogeneic fetus to be tolerated within the maternal environment.