Objective: Platelet aggregometry was used to discriminate platelet sensitivity to prostaglandin E1 (PGE1) inhibition, to evaluate whether platelet behaviour in pre-eclamptic women was different in this respect than from that in nonproteinuric hypertensive women. Methods: The amount of PGE1 required to inhibit in vitro platelet aggregation induced by arachidonic acid was determined in samples from 60 women: 20 nonpregnant controls, 20 women with normal pregnancies, 10 women with gestational hypertension and 10 with pre-eclampsia. Results: The response to arachidonic acid was similar among the four groups. Amounts of PGE1 necessary to inhibit platelet aggregation were significantly higher in normal pregnant women compared with nonpregnant controls (P < 0.001). Platelets from pre-eclamptic women required significantly higher concentrations of PGE1 to inhibit aggregation than the other groups studied (P < 0.001). However, there was no significant difference between normal and nonproteinuric hypertensive pregnant women. Conclusions: Our findings support the notion that increased platelet reactivity during late pregnancy is exacerbated in pre-eclamptic women but not in nonproteinuric hypertensive women. This is in agreement with the hypothesis that pre-eclampsia and gestational hypertension are different conditions. Prospective studies are required to confirm if this simple test may be useful in the early identification of pregnant women at risk for pre-eclampsia.