BACKGROUND: Losartan potassium, an orally active, highly selective AT (1) angiotensin II receptor inhibitor, effectively reduces blood pressure by directly blocking receptors. The purpose of this study is to compare the antihypertensive efficacy, safety and tolerability of losartan potassium and fosinopril in patients with stage 1 to 3 hypertnesion. METHODS: In this comparative, open labelled, randomized, parallel study, the efficacy, safety and tolerability of once-daily losartan (50mg) versus once-daily fosinopril (10mg) were evaluated over twelve weeks in 96 patients with stage 1-3 hypertension. If trough sitting diastolic blood pressure was equal to or greater than 90mmHg after a 6 week treatment period, the dosage for both study drugs was doubled until the end of the study (week 12). RESULTS: After 6 weeks of treatment, mean reductions in trough sitting diastolic blood pressure were 7.4mmHg (95% confidence interval 5.0-9.9) with 50mg losartan and 6.7mmHg (95% confidence interval 4.4-9.2)with 10mg fosinopril. After 12 weeks of treatment (after dose titration), mean reductions in trough sitting diastolic blood pressure were 9.4mmHg (95% confidence interval 6.7-12.1) with losartan and 10.3mmHg (95% confidence interval 7.6-12.9) with fosinopril. At weeks 6 and 12, there were no dignificant difference in the mean reduction of through ditting diastolic blood pressure between the losartan group and the fosinopril group. Losartan and fosinopril were well tolerated without significant clinical and laboratory adverse reactions. The incidence of dry cough was lower in the losartan group than in the fosinopril group but not statistically significant. CONCLUSIONS: The antihypertensive effect of once-daily administration of losartan is similar to that of once-daily administration of fosinopril in patients with hypertension. Both losartan and fosinopril are well tolerated without significant adverse reaction.