The study was designed to assess the antihypertensive effect of combined angiotensin-converting enzyme (ACE) inhibition and angiotensin II type 1 receptor (AT1) antagonism in patients with essential hypertension. Twenty patients with uncontrolled ambulatory diastolic blood pressure (BP) after 6 weeks of ACE inhibitor monotherapy (benazepril, 20 mg, o.d.) were randomized to receive double-blind valsartan, 80 mg, o.d. (AT1 antagonist) or matching placebo for 5 weeks while continuing to receive background benazepril. Then patients crossed over to the alternative regimen for a second 5-week period. The 24-h ambulatory BP was monitored on the final day of the benazepril monotherapy period and on the final day of each double-blind treatment period. Valsartan added to benazepril produced a significant antihypertensive effect with a benefit over placebo of 6.5 +/- 12.6/4.5 +/- 8.0 mm Hg (systolic/diastolic) for average awake ambulatory BP (p < 0.05), 7.1 +/- 9.4/5.6 +/- 6.5 mm Hg for asleep BP (p < 0.01), and 6.8 +/- 9.7/4.9 +/- 6.8 mm Hg for average 24-h ambulatory BP (p < 0.01). Pulse rate was unaffected. Plasma active renin was higher on the benazepril-valsartan combination compared with benazepril-placebo (p < 0.05). There was no change in routine biochemical variables when valsartan was added to benazepril. Six patients reported mild dizziness or fatigue (three also with placebo). These data suggest that in hypertensive patients uncontrolled with an ACE inhibitor, the addition of an AT1 antagonist provides a powerful and safe antihypertensive drug combination.