Severe hypertension is difficult to control. This prospective, randomized, double-blind, active-controlled, multicenter trial compared efficacy and safety of once-daily irbesartan/hydrochlorothiazide (HCTZ) combination therapy with irbesartan monotherapy in severe hypertension. Patients who were untreated or uncontrolled on monotherapy (seated diastolic blood pressure [BP] > or =110 mm Hg) received fixed-dose irbesartan 150 mg/HCTZ 12.5 mg combination therapy for 7 weeks, force-titrated to irbesartan 300 mg/HCTZ 25 mg at week 1 (n=468); or irbesartan 150 mg monotherapy, force-titrated to 300 mg at week 1 (n=269). Significantly more patients on combination therapy achieved seated diastolic BP <90 mm Hg at week 5 (primary end point) compared with monotherapy recipients (47.2% vs 33.2%; P=.0005). Likewise, significantly more patients attained goals per the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) (<140/90 mm Hg) at week 5 (34.6% vs 19.2%, respectively; P<.0001), while the mean difference between combination and monotherapy in seated diastolic BP and seated systolic BP was 4.7 mm Hg and 9.7 mm Hg (P<.0001). Greater and more rapid BP reduction with irbesartan/HCTZ was achieved without additional side effects.