Background: Irbersartan, an orally active antihypertensive agent, effectively reduces blood pressure by blocking angiotensin II receptors without any significant adverse effects. The purpose of this study is to compare the antihypertensive efficacy, safety and tolerability of irbesartan and enalapril in hypertensive patients. Methods: In this two centers, double-blind, randomized, non-inferiority study, the efficacy, safety and tolerability of once-daily irbesartan(150mg) versus once-daily enalapril(10mg) were evaluated over 8 weeks in 67 patients who had diastolic pressure between 95mmHg and 114mmHg on two measurements. If trough sitting diastolic blood pressure was equal to or greater than 90mmHg after a 4-week treatment period, the dosage for both study drugs was doubled until the end of the study. Baseline pressures, antihypertensive effect, side effects, laboratory findings were compared before and after treatment. Results: Data from 57 of 67 patients were eligible for intention to treat analysis. After the 8 weeks treatment with dose titration, mean reductions in peak sitting diastolic blood pressure were 11.9mmHg(95% confidence interval 7.61 -16.13) with irbesartan and 10.9mmHg$(95%;confidence;interval;7.05{sim}14.70)$ with enalapril. There was no significant difference between the two treatments in the percentage of patients achieving an effective blood pressure reduction or in the degree of change in mean systolic and/or diastolic blood pressure. Safety profiles were also similar between treatments. Conclusions: The antihypertensive efficacy of once-daily administration of irbesartan is non-inferior to that of enalapril in hypertensive patients. Both irbesartan and enalapril are well tolerated with similar safety profiles.