To document maternal central hemodynamics during the preclinical and clinical phases of nonproteinuric gestational hypertension and preeclampsia. We conducted a longitudinal study of 400 primigravidas who were monitored throughout pregnancy using Doppler echocardiography. Multinomial logistic regression was used to identify variables associated with risk of hypertension. Gestational hypertension developed in 24 women and preeclampsia developed in 20. Compared with normotensive controls, women who had preeclampsia had significantly elevated cardiac outputs before clinical diagnosis, but total peripheral resistance was not significantly different during this latent phase. During the clinical phase of preeclampsia, there was a marked reduction in cardiac output and increase in peripheral resistance. All women who had gestational hypertension had significantly elevated cardiac outputs before and during the clinical course of the condition. Our data support the concept of a hyperdynamic disease model for preeclampsia, with a subsequent hemodynamic crossover to low cardiac output and high resistance circulation coinciding with the onset of the clinical syndrome. Women with gestational hypertension had no such hemodynamic crossover and maintained hyperdynamic circulation throughout pregnancy.