In order to investigate the efficacy and safety of oral fosinopril, a new phosphorus containing angiotensin converting enzyme inhibitor, a single dose of 10 to 20mg was administered in 23 hypertensive patients with diastotic blood pressure above 95mmHg and all other anti-hypertensive agents were not administered during 4 weeks of study. Blood pressure and heart rate were measured on the 2nd and 4th week of therapy. The complete blood count with platelet count, blood chemistry by SMA-12 and serum electrolytes were performed at the begining and 4th week of therapy. The urinalysis and electrocardiography were performed at the beginning and 4th week of therapy. Any kinds of side effects were actively questioned by the examining physicians. The following results were obtained : 1) At the beginning and 4th weeks of therapy, the average systolic and diastolic pressure were 170.0±17.6/101.6±6.1mmHg, 142.7±15.1/87.3±6.7mmHg respectively. The systolic and diastolic blood pressure were declined statistically significantly(p<0.05) throughout the period of treatment and diastolic blood pressure of all subjects except 3 patients(86%) was maintained below 90mmHg after 4th week of treatment. 2) There was no significant change in the pulse rate before and after therapy. 3) There were no significant changes in blood chemistry, serum electrolytes, hematologic findings, urinalysis and electrocardiographic findigns. 4) side effect were developed in 5 patients(23%) with dry cough, 3 patients(13%) with headache and 2 patients with facial edema but side effects were mostly mild in nature without potenitally serious episodes. These results suggested that antihypertensive therapy with onec-daily fosinopril was effective and well tolerated in essential hypertensive patients.