The ability of low-dose aspirin therapy to prevent preeclampsia is controversial. The 19 randomized, placebo-controlled trials of low-dose aspirin therapy reported in the literature were categorized according to the risk factors of the women studied–nulliparity, underlying medical illness, poor obstetric history, and multiple gestation. Low-dose aspirin therapy reduced the incidences of preeclampsia among women with poor obstetric histories and among high-risk nulliparous women but was ineffective among women with underlying medical illness. It was marginally effective among low-risk nulliparous women, and benefits for women with multiple gestations are unclear. More research is needed to better identify high-risk nulliparous women who might benefit from the use of low-dose aspirin therapy and to define potential benefits for women with multiple gestations. The differential effects of low-dose aspirin therapy in the various risk groups are probably a result of varying roles in the groups of abnormal arachidonic acid metabolism in mediating preeclampsia. It is premature to abandon the use of low-dose aspirin therapy for preeclampsia prevention. (Am J Obstet Gynecol 2000;183:523-8.)