Fetal levels of a representative lipid peroxide malondialdehyde (MDA) were measured as an index of oxidative stress in pregnancies complicated by preeclampsia (n = 19) and in normal pregnancy (n = 23). PO2, PCO2 and pH were also measured in umbilical venous blood obtained by cordocentesis. Plasma MDA levels rose an average of 26% above those of normal pregnancy (p < 0.05) and the elevation correlated inversely with umbilical venous PO2 and pH. The results indicate that fetal oxidative stress occurs in preeclampsia before the onset of labor.