BACKGROUND: To evaluate the safety and the efficacy of amlodipine, a dihydropyridine calcium antagonist, monotherapy in the treatment of moderate essential hypertension. METHOD: Amlodipine 5mg once a day was administered as a starting dose in 30 patients with essential hypertension in the morning and a one step upward titration was performed (amlodipine 10 mg once a day) was done at the end of 4weeks treatment. Final evaluation was done at 12weeks with laboratory test and echocardiogram. RESULT: Within 4weeks treatment with dose of 5mg amlodipine once a day, the systolic blood pressure (SBP) was decreased(184.5+/-23.3/150.5+/-16.0mmHg,p<0.000), and the diastolic blood pressure(DBP) was also decreased significantly (109.9+/-04.6/92.3+/-11.5mmHg, P<0.001). After 12 weeks of treatment with a mean dosage of 6.6mg once a day, SBP and DBP was maintained comparing with basal level (147.0+/-15.8/88.1+/-0.9mmHg, respectively). The efficacy of amlodipine treatment was noted an excellent in 16 patients(53.3%), good in 4 patient(13.3%), fair in 4 patients(13.3%), and failed in 2 patients(6.7%). There was no significant change in heart rate before and after amlodipine treatment. (80.0+/-2.3/80.9+/-10.4 beats/minute n.s). Amlodipine had not significant effects on laboratory findings such as serum creatinine, BUN, ALT/AST, hemoglobin, leukocyte count,platelet and lipid profiles. There was facial flushing 2 patients, but no need to discontinue administration of amlodipine and all patients completed for 12weeks therapy. CONCLUSION: It is concluded that amlodipine is an effective antihypertensive agent, as monotherapy once a day in patients with moderate essential hypertension.