The authors assessed the early antihypertensive efficacy of olmesartan medoxomil (OM) in a 12-week prospective observational study. Of 2221 patients with untreated hypertension who received OM (mainly 10 or 20 mg), 331 patients whose blood pressure (BP) was measured at 1 week after initiation of treatment were defined as the “early BP determination group,” whereas the remaining 1890 patients were defined as the “standard BP determination group.” Baseline characteristics, doses of OM, concomitant drugs used, and BP during treatment did not differ between the 2 groups. The achievement rate of BP target (<140/90 mm Hg) was 28.4% at 1 week in the early BP determination group and 28.3% at 2 weeks in the standard BP determination group (P=NS). Rates of adverse drug reactions in the 2 groups were similar. The present study suggests that OM is safe and effective in reducing BP at an early time point of treatment.