Twenty-three patients older than 50 years, with systolic hypertension, underwent hemodynamic study. Cardiac output and stroke volume varied widely, with several high values. An elevated systemic vascular resistance, when considered as a function of the cardiac output, was the most prevalent hemodynamic abnormality. After administration of hydrochlorothiazide, 50 mg/day, for one month in 20 patients, 18 had a significant fall in BP, systemic vascular resistance, and stroke volume. After one year of continuous therapy, the hemodynamics, studied in 14 patients, did not change further. There were no clinical difficulties with carbohydrate intolerance, azotemia, hyperuricemia, or hypokalemia. No patient had symptoms of orthostatic hypotension or cardiac arrhythmias. Thus, thiazide therapy effectively and safely lowers the BP in most patients with systolic hypertension by reducing systemic vascular resistance.