To determine the rate and extent of reversal of left ventricular (LV) hypertrophy, eight patients with moderate to severe essential hypertension (diastolic blood pressure 116 mmHg, s.d. = 8) underwent serial echocardiography during 12 months of closely supervised antihypertensive therapy with multiple drugs. LV cavity size and average wall thickness were measured by sector-directed M-mode echocardiography. LV mass was estimated from a formula which had previously been validated. All subjects had increased LV mass (greater than 125 g/m2). From 6-12 months, 84% of standing diastolic blood pressures, measured monthly, were 90 mmHg or less. During therapy LV mass decreased from 206 g/m2 (s.d. = 98) initially to 168 g/m2 (s.d. = 82) after 6 months (P less than 0.001) with no further significant change by 12 months. In four patients with less severe hypertrophy, LV mass was within the normal range after 6 months of therapy. In four patients with more severe hypertrophy, LV mass remained significantly elevated at 6 and 12 months. Severe hypertensive LV hypertrophy may be only partially reversible, despite good blood pressure control.