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.