Preeclampsia is a disease unique to pregnancy that contributes substantially to maternal and fetal morbidity and mortality. The condition has been thought to be one of hypoperfusion in which increased vascular resistance characterizes the associated hypertension. This study was designed to test an alternative hypothesis, that preeclampsia is characterized by high cardiac output. In a blinded longitudinal study of nulliparas with uncomplicated pregnancies, cardiac output was measured serially by Doppler technique. Cardiac output was elevated throughout pregnancy in patients who became preeclamptic (P = .006). Six weeks postpartum, the hypertension of the preeclamptic subjects had resolved but cardiac output remained elevated (P = .001) and peripheral resistance remained lower than in the normotensive subjects (P = .001). This study demonstrates that preeclampsia is not a disease of systemic hypoperfusion and challenges most current models of the disease based on that assumption.