A clinical trial was done to evaluate the antihypertensive efficacy and side effects of manidipine, a new calcium antagonist, in 30 patients with mild to moderate essential hypertension. 1) The study patients consisted of 19 men and 11 women, and the mean age was 51.8 years. 2) Blood pressure dropped significantly in 2 weeks and in 4 weeks, and well maintained throughout the study period. The mean-pressure drop was 26.2/14.9mmHg after 10 weeks. 3) Heart rate did not change significantly with manidipine therpy. 4) Optimal dose for effective pressure-drop was between 10 and 20 mg in 86% of patients. Overall good antihypertensive effect was achieved in 83% of patients. 5) All of the laboratory parameters including blood chemistry, glucose, lipid and electrolytes did not change, but serum calcium increased from 9.2% mg/dl (p=0.001) in 10 weeks. 6) Side effects were mild in nature(palpitation in 3, dry mouth in 1, weakness in 1 and impotence in 1 patient). In conclusion, manidipine monotherapy with 10 to 20 mg once a day regimen is effective and well tolerated in the patients with mild to moderate essential hypertension.