The effect of ketanserin, serotonin antagonist, among 19 korean patients over 55 years with essential hypertension was assessed in an open clinical trial for three months. patients were given Ketanserin 20mg bid with monthly follow-up visits. Mean values of systolic/diastolic blood pressures fell from 169+/-17/104+/-10mmHg to 155+/-14/94+/-9mmHg at 2 weeks(p<0.01) and to 147+/-10/87+/-6mmHg at end of treatment 12 weeks after(p<0.001). There was no significant change in heart rate. Transient mild side effects were observed in 5 patients. We conclude that Ketanserin is an effective and safe drug for the treatment of elderly hypertensives.