In an open clinical study, the efficacy and safety of carvedilol was investigated in 26 severely hypertensive patients controlled inadequately on a diuretic [diastolic blood pressure (DBP) greater than 120 mm Hg at first visit and greater than 110 mm Hg following more than 1 week administration of a diuretic]. Following diuretic treatment all patients were initially administered 5 mg of carvedilol once daily. The dose was gradually increased to 10 mg and 20 mg until DBP was reduced below 100 mm Hg or until it was reduced by at least 10 mm Hg. Antihypertensive activity of carvedilol (5 mg) was sufficient in only three cases, but after 4 weeks (inpatients) or 8 weeks (outpatients) administration of carvedilol (10 mg or 20 mg), DBP/systolic blood pressure was significantly reduced from 176 +/- 6/117 +/- 3 to 145 +/- 3/94 +/- 2 mm Hg (p less than 0.001) in all patients. Overall, a sufficient antihypertensive effect was observed in 80% of the patients. Heart rate was significantly decreased from 76 +/- 2 to 67 +/- 2 beats/min, but no patient experienced bradycardia. Carvedilol was generally well tolerated. These findings suggest that 10-20 mg of carvedilol once daily, in combination with a diuretic, is an effective and safe treatment for patients with severe hypertension.