Diuretic-based therapy is less effective in reducing the cardiac complications of hypertension than the risk of stroke and may be less effective in reducing left ventricular (LV) mass than is therapy with angiotensin converting enzyme (ACE) inhibition. In view of the strong association of LV hypertrophy with cardiovascular risk, this study was designed to compare the impact of therapy with a diuretic and ACE inhibition on cardiac and vascular structure. Fifty essential hypertensives (74% male, 88% nonwhite) participated in a double-blind study for 6 months and were randomized to either ramipril or hydrochlorothiazide (HCTZ). Echocardiography, carotid ultrasonography, and ambulatory blood pressure (BP) monitoring were performed at baseline and 3 and 6 months after initiation of therapy. The 22 ramipril patients were comparable to the 28 HCTZ patients at baseline in age, race, and 24-h BP. Although HCTZ resulted in a greater reduction in 24-h BP, only treatment with ramipril resulted in a decrease in LV mass (193 to 179 g, P < .005, v 184 to 182 g, P = NS), attributable to a reduction in wall thicknesses but not in chamber diameter. In multivariate analysis, both change in BP and treatment group were independent predictors of change in LV mass. Importantly, although neither drug reduced carotid artery cross-sectional area, relative wall thickness increased due to a tendency for vessel diameter to decrease and wall thickness to increase, particularly in the diuretic group. Ramipril caused a sustained fall in plasma angiotensin II, whereas HCTZ increased angiotensin II levels. Although diuretic therapy was more effective in lowering ambulatory BP in this predominantly nonwhite population, only therapy with ACE inhibition was associated with regression of LV mass. Vascular geometry was altered consistent with the reduction in distending pressure resulting in vascular remodelling.