Abstract--This study investigated whether production of reactive oxygen species by neutrophils differs between women with preeclampsia and those with essential hypertension. First, we assessed superoxide production by neutrophils during pregnancy and 4 weeks after delivery in 15 healthy pregnant women, 12 women with preeclampsia, and 7 pregnant women with essential hypertension. We then examined effects of serum from each subject on superoxide production by neutrophils obtained from healthy nonpregnant women. Neutrophil superoxide production was measured by cytochrome C reduction. N-formyl-methionyl-leucyl-phenylalanine-stimulated superoxide production was significantly increased in neutrophils from women with preeclampsia and women with essential hypertension compared with normal pregnant women. Four weeks postpartum, the level of superoxide production was significantly decreased in women with preeclampsia but not in women with either normal pregnancy or essential hypertension. When neutrophils obtained from nonpregnant women were preincubated with predelivery sera from each group, sera from women with preeclampsia significantly enhanced superoxide production compared with sera from the other 2 groups. When postpartum serum was used, enhancement of neutrophil superoxide production by serum from women with preeclampsia was significantly decreased compared with that by predelivery serum. In conclusion, increased neutrophil superoxide production resolved after delivery in preeclampsia, whereas activation persisted postpartum in women with essential hypertension. The different transition of neutrophil superoxide production in response to pregnancy appears to be that preeclampsia is characterized by the presence of serum factors that enhance neutrophil superoxide production. Thus, in preeclampsia, serum factors bear a more essential role producing superoxide than a behavior of neutrophils.