The pathophysiology of preeclampsia involves profound systemic vasoconstriction. Its etiology may be related to reduced blood flow to the placenta, leading to the elaboration of soluble, vasoactive factors which increase maternal systemic vascular resistance and cause renal dysfunction. Reduced bioactivity of vascular endothelial growth factor (VEGF) may play a central role in this pathophysiology. Previous clinical studies have strongly suggested that relaxin is a systemic and renal vasodilator. In nonclinical studies, relaxin administration to monkeys has been associated with increased vessel density in the endometrium, and in previous human trials relaxin administration has been highly correlated with increased menstrual bleeding in women. VEGF has been proposed as a mechanism for these endometrial effects. Together, these data suggest that relaxin may be able to relieve systemic and renal vasoconstriction and improve placental perfusion in women with preeclampsia. As a first step in the development program for relaxin in this indication, a multicenter, randomized, double-blind, placebo-controlled phase I safety study in women with severe preeclampsia has been launched in the USA. Three doses of relaxin, 3, 10, and 30 μg/kg of body weight/day, or placebo, will be administered for up to 72 h in women admitted to the hospital for management of their disease. Although the trial is primarily focused on safety, signs of efficacy, such as changes in blood pressure and renal markers, will also be assessed.