In general, treatment with most angiotensin receptor blockers (ARBs) increases plasma angiotensin II (Ang II) level because of a lack of negative feedback on renin activity. Olmesartan is a potential ARB inducing activation of angiotensin-converting enzyme 2 (ACE2) that hydrolyzes Ang II to Ang 1-7, and has shown a beneficial effect on ventricular remodeling. Indeed, a previous study reported that olmesartan treatment resulted in decreased plasma levels of Ang II and aldosterone. However, there has not yet been a study showing the relationship of chronic effects of olmesartan on Ang II and the left ventricular mass index (LVMI) in comparison with those of other ARB.A total of 50 stable outpatients with essential hypertension who had received candesartan for more than 1 year were randomized into two groups: control group (n=25): continuous candesartan treatment at a stable dose; and olmesartan group (n=25): candesartan (8 mg day(-1)) was changed to olmesartan given at a dose of 20 mg day(-1). There was no difference in the baseline characteristics between the two groups. In the control group, there were no significant changes in blood pressure, LVMI or biomarkers during 12 months of study. In the olmesartan group, blood pressure did not change and plasma levels of Ang II decreased during 12 months of study, whereas LVMI was significantly decreased after 12 months (135+/-36 vs. 123+/-29 g m(-2); P<0.01).These findings indicate that replacing candesartan with olmesartan decreased LVMI in association with a sustained decrease of plasma Ang II over a 12-month period without changing blood pressure or plasma aldosterone in patients with essential hypertension.