Abstract Objective: To determine if assessment of maternal hemodynamics could predict women at risk for the development of preeclampsia, if treatment directed at hemodynamic abnormalities before the onset of hypertension could prevent preeclampsia, and if mothers could be treated in a way that protects fetal growth. Methods: A double-blinded, randomized controlled trial was conducted. Subjects were considered to be at risk for preeclampsia if their cardiac output was greater than 7.4 L/min before 24 weeks’ gestation. Nulliparous and diabetic subjects at risk were treated with 100 mg of atenolol or placebo. Cardiac output was measured by Doppler technique. Inulin and para-aminohippurate clearances were performed. Results: Treatment with atenolol reduced the incidence of preeclampsia from 5 of 28 (18%) to 1 of 28 (3.8%), ( P = .04). Nulliparous women determined to be at risk for preeclampsia were similar to diabetic women at risk. Each was significantly heavier and had inulin and para-aminohippurate clearances greater than the control group. Treatment with atenolol was associated with infants weighing 440 g less than infants in the nulliparous placebo group, ( P = .02). No effect on birth weight was seen in the diabetic patients. Mothers of the smallest infants who were treated with atenolol could be identified by unexpectedly large reductions in cardiac output. Conclusion: Measurement of cardiac output in the second trimester identified women at risk for preeclampsia. Treatment with atenolol decreased the incidence of preeclampsia. Nulliparous and diabetic women at risk for preeclampsia were similar with regard to maternal hemodynamics, maternal weight, and renal function. Treatment with atenolol was associated with reduced infant birth weight.