The above study was undertaken in order to demonstrate the efficacy of ketanserin in mild to moderate essential arterial hypertension. Twenty-two patients, mean age 49.7 yrs., were studied during 36 months. After a 15-day washout period with clinical and instrumental assessment of basal conditions, treatment with 40 mg ketanserin every 12 hrs. was started. Pressure, blood chemistry and instrumental investigation were carried out systematically and showed the hypotensive action of ketanserin as well as a significant (about 12%) rise in HDL-cholesterol.