Blood pressure (BP) reductions with agents that block the renin-angiotensin system are regarded as less effective as monotherapy in African Americans than other ethnic groups. This practice-based study compares the efficacy of an angiotensin receptor blocker-based regimen in African-American and Caucasian patients. Included in the 10-week study were 173 African-American and 1296 Caucasian patients. Efficacy was based on differences in 24-hour ambulatory BP. After baseline ambulatory BP monitoring and office BPs were obtained, all patients were started or switched to the angiotensin receptor blocker telmisartan, 40-80 mg daily, plus hydrochlorothiazide 12.5 mg daily (if needed for office BP control: <140/90 mm Hg). More African Americans required the addition of a low-dose thiazide diuretic than Caucasians (47.3% vs. 34.9%; p=0.021). Once patients with white coat hypertension were excluded (i.e., those with baseline ambulatory BP monitoring <130/80 mm Hg), ambulatory BP monitoring changes were similar between groups. A greater proportion of African Americans than Caucasians without white coat hypertension also needed combination therapy (52.1% vs. 39.5%; p=0.04). While achievement of BP goal was similar between groups by office criterion (<140/90 mm Hg), differences were noted by ambulatory BP monitoring (<130/80 mm Hg) (48.0% in African American vs. 63.2% in Caucasians; p=0.01) despite the same BP reductions, reflecting higher baseline values in African Americans. We conclude that an angiotensin receptor blocker as part of a BP-lowering strategy is effective in previously untreated African-American patients, although a higher proportion will require the use of a diuretic compared with Caucasians.