BACKGROUND The antihypertensive efficacy and safety of losartan, a specific and selective angiotensin II (AII) receptor antagonist, was compared to captopril in patients with mild or moderate essential hypertension. METHODS This multinational, randomized trial consisted of a 4-week single-blind, placebo baseline period followed by a 12-week double-blind, parallel comparison of once-daily administration of losartan 50 mg or twice-daily administration of captopril 25 mg. After 6 weeks of treatment, the daily dosage was doubled in patients whose sitting diastolic blood pressure (SiDBP) remained > or = 90 mm Hg. METHODS Patients with essential hypertension having a mean trough SiDBP of 95-115 mm Hg after the placebo baseline period were randomized to losartan (N = 192) or captopril (N = 204) treatment. METHODS The primary efficacy variable was the mean change from baseline to Week 12 in trough SiDBP. Safety was assessed by recording spontaneously reported or observed adverse experiences and clinical laboratory measurements. RESULTS After 12 weeks, both treatments produced clinically important reductions in trough SiDBP and sitting systolic blood pressure (SiSBP). These mean reductions (SiDBP, SiSBP) were significantly greater in the losartan group (-11.5 and -15.4 mm Hg, respectively) than in the captopril group (-9.3 and -12.2 mm Hg, respectively) (p = 0.010 for diastolic and p = 0.023 for systolic). The percentage of patients exhibiting an excellent (trough SiDBP < 90 mm Hg) or good (trough SiDBP > 90 mm Hg, with decrease of > or = 10 mm Hg) antihypertensive response to losartan and captopril therapy at Week 12 was comparable (60.0% and 54.7%, respectively). The percentage of patients reporting a clinical adverse experience considered drug-related by the investigator was 13% in the captopril group and 10% in the losartan group. The incidence of drug-related cough was 2.6% in the losartan group and 4.4% in the captopril group. CONCLUSIONS Once daily administration of losartan 50 to 100 mg is an effective treatment for patients with essential mild to moderate hypertension. The antihypertensive efficacy of losartan 50/100 mg is significantly greater than that of twice daily captopril 25/50 mg. Both treatments were generally well-tolerated. The number of patients with the side effect of cough was higher following captopril.