Patients with resistant hypertension are at increased risk for cardiovascular events. The addition of new treatments to existing therapies will help achieve blood pressure (BP) goals in more resistant hypertension patients. In the current trial, 849 patients with resistant hypertension receiving ≥3 antihypertensive drugs, including a diuretic, at optimized doses were randomized to the selective endothelin A receptor antagonist darusentan, placebo, or the central α-2 agonist guanfacine. The coprimary end points of the study were changes from baseline to week 14 in trough, sitting systolic BP, and diastolic BP measured in the clinic. Decreases from baseline to week 14 in systolic BP for darusentan (−15±14 mm Hg) were greater than for guanfacine (−12±13 mm Hg; P<0.05) but not greater than placebo (−14±14 mm Hg). Darusentan, however, reduced mean 24-hour systolic BP (−9±12 mm Hg) more than placebo (−2±12 mm Hg) or guanfacine (−4±12 mm Hg) after 14 weeks of treatment (P<0.001 for each comparison). The most frequent adverse event associated with darusentan was fluid retention/edema at 28% versus 12% in each of the other groups. More patients withdrew because of adverse events on darusentan as compared with placebo or guanfacine. We conclude that darusentan provided greater reduction in systolic BP in resistant hypertension patients as assessed by ambulatory BP monitoring, in spite of not meeting its coprimary end points. The results of this trial highlight the importance of ambulatory BP monitoring in the design of hypertension clinical studies.