Patients with hypertension exhibit changes in vessel conductance and resistance. The aim of this study was to evaluate the effect of the angiotensin receptor blocker irbesartan on coronary microvascular function. Thirty-six hypertensive patients without coronary artery or systemic disease were examined. Coronary flow velocity reserve (CFR) was measured using transthoracic Doppler echocardiography in 18 men (54±9 years) before and after 3 months of treatment with 600 mg/d of irbesartan and in 18 controls (55±11 years). Carotid intima-media thickness (IMT) was evaluated with high-resolution echocardiography. Baseline CFR did not differ between groups. CFR significantly improved in the irbesartan group (from 2.87±.42 to 3.78±.32; P<.001), but remained unchanged in controls (from 2.94±.61 to 3.06±.72; P=not significant). CFR improved with treatment independent of associated risk factors. BP decreased from 150±18 mm Hg to 129±25 mm Hg (P<.001) during treatment, whereas IMT and left ventricular mass index showed no significant differences at the end of the follow-up period in both groups. Three-month irbesartan treatment significantly increased CFR in patients with hypertension. This improvement is attributed to blockade of the renin-angiotensin system. Coronary microvascular function was shown to improve independent of hypertrophy regression. Patients with lower baseline CFR tended to show a more pronounced CFR response. J Clin Hypertens (Greenwich). 2011;13:155–161. © 2010 Wiley Periodicals, Inc.