BACKGROUND: The efficacy of oral angiotensin converting enzyme(ACE) inhibitors in the treatment of hypertension has been well documented. Fosinopril is a member of a new class of phosphorous containing ACE inhibitors and is a cleared almost equally by both renal and hepatic routes. METHODS: To evaluate the effects of fosinopril, a new angiotensin converting enzyme inhibitors, in patients with mild to moderate essential hypertension, we administered fosinopril 10~40mg/day once daily to 32 patients(14 males, 18 females; range of age, 40~71 years old) with dose titration every 2 weeks. RESULTS: The baseline blood pressure after 4 weeks wash-out period was 156.3+/-22.3/104.8+/-5.93mmHg. After fosinopril therapy, the blood pressure declined to 150.0+/-27.8/96.4+/-12.2mmHg with 10mmg/day and 144.4+/-20.0/95.21+/-9.95mmHg with 20mg/day, 139.6+/-18.3/85.1+/-9.71mmHg with 30mg/day.During the period of maintenance after dose titration of fosinopril every 2 weeks for 8 weeks, the blood pressure were declined statistically significant (p<0.05). But the heart rate was not changed throughout the period of treatment. The efficacy of fosinopril therapy disclosed 26 patients(81.3%) with normalized diastolic blood pressure and 4 patients(11.8%) with more than 10mmHg decline of diastolic blood pressure. Therefore, the efficacy of fosinopril in these particular patients was 93.1% and two patients(6.9%) showed a failure of fosinopril therapy. The adverse reaction due to fosinopril were reported in 3 patients(9.4%) with dry cough, 1 patients(3.1%) with palpitation and 1 patients(3.1%) with dizziness, but there were patients who discontinued fosinopril due to adverse reaction. CONCLUSION: These results suggested that antihypertensive therapy with fosinopril was effective and well tolerated in patients with mild to moderate essential hypertension.