Objective: In a previous study we showed neutrophil platelet-activating factor (PAF) to be reduced in preeclampsia, while plasma lyso-PAF was increased. The aim of the current study was to examine (PAF) acetyl hydrolase activity (the enzyme that degrades PAF) in plasma and urine of new groups of preeclamptic, hypertensive, and normal pregnant women, to see if alterations in the activity of this enzyme might explain the changes in neutrophil PAF and plasma lyso-PAF previously seen in severe preeclampsia.Methods: Plasma and urinary acetyl hydrolase activity was measured in 22 women with proteinuric preeclampsia and 22 age- and gestational-age-matched normal pregnant women. To determine whether alterations in acetyl hydrolase activity in preeclampsia were due to hypertension per se, a comparison was made to a group of 10 age- and gestational-age-matched pregnant women with uncomplicated essential hypertension. The pregnant women were studied prior to delivery and at 6 weeks and 6 months postpartum, and were also compared to a group of 17 age-matched nonpregnant controls.Results: Plasma acetyl hydrolase activity was elevated in the preeclamptic and essential hypertensive women at all visits. However, the elevation in these two groups was most pronounced in the postpartum period. The elevation in plasma acetyl hydrolase was accompanied by increases in the levels of LDL (low-density lipoprotein)-cholesterol (P <0.05) in the same groups over the same time period. Urinary acetyl hydrolase activity was elevated in women with preeclampsia (P <0.01) compared to all other groups before delivery, and was closely related to the amount of proteinuria.Conclusion: Increases in plasma acetyl hydrolase activity in the preeclamptic and essential hypertensive groups are most pronounced after delivery, suggesting that this elevation represents a constitutional difference between the two groups who were hypertensive during pregnancy and those who were normotensive throughout. The increase in urinary acetyl hydrolase is specific to preeclampsia and resolves after delivery, but it is not known whether it is the result of a plasma leak or reflects altered renal PAF synthesis.