It has been proposed that endothelial cell activation is the primary event in the multisystem disorder of preeclampsia. Evidence for endothelial involvement in this condition abounds. The best-characterized morphologic abnormality of this syndrome, glomerular endotheliosis, involves endothelial cells. Also associated with preeclampsia is a loss of endothelial cell integrity, with the consequent increase in vascular permeability, and an increase in the circulating levels of the endothelial cell markers, fibronectin, von Willebrand factor, tissue plasminogen activator, and plasminogen activator inhibitor-1. It is now well documented that endothelial activation contributes to the coagulation abnormalities observed in this disease. There is much evidence that the endothelial alterations in preeclampsia result from one or more circulating factors. The incubation of cultured endothelial cells with serum or plasma samples, taken from normal pregnant women and women with preeclampsia, results in marked alterations in cell behavior and metabolic processes. More recently, experiments employing myographic techniques have demonstrated convincingly the effects of a circulating factor(s) on the function of endothelial cells of resistance arteries. Vascular endothelial growth factor (VEGF) possesses many of the characteristics required of a candidate circulating factor. It contains a hydrophobic secretory signal sequence, exerts in vitro effects specific to vascular endothelial cell, and promotes endothelial expression of procoagulant activity. Circulating VEGF concentrations are elevated in women with preeclampsia, and VEGF increases microvascular endothelial cell prostacyclin production in a dose-dependent manner, analogous to the acute effects of plasma from patients with preeclampsia. Similarly, in myographic studies, when myometrial resistance arteries are incubated with VEGF, there are dose-dependent alterations in endothelium-dependent behavior, mirroring those found after incubation with plasma from patients with preeclampsia.