The antihypertensive efficacy of once- and twice-daily losartan was evaluated in a randomized, double-blind, placebo-controlled, parallel-group trial using ambulatory 24-h blood pressure monitoring in 122 patients with mild to moderate essential hypertension. Data are reported for 112 patients who completed the active treatment phase. Losartan exerted a sustained 24-h antihypertensive effect at all doses studied (50 or 100 mg once daily, or 50 mg twice daily). Losartan demonstrated a smooth, gradual onset of action, did not affect normal circadian variations on blood pressure, and had no significant effect on heart rate. An additive antihypertensive effect was obtained by adding low-dose hydrochlorothiazide to losartan monotherapy at all doses of losartan studied. Losartan was well tolerated during the study with a frequency of adverse events comparable with placebo. No major adverse events were recorded. The results of this study indicate that losartan is an effective and well-tolerated antihypertensive agent that provides 24-h blood pressure-lowering efficacy from a single daily dose of 50 mg.