To assess the reversibility of left ventricular hypertrophy (LVH) during terazosin therapy, we studied changes in LVH as determined by echocardiography and humoral parameters before and after three and 12 months of terazosin monotherapy in ten patients. Blood pressure decreased within four weeks of treatment and the antihypertensive effect was sustained throughout 12 months. Left ventricular mass index decreased significantly from 126 +/- 22 g/m2 to 109 +/- 24 g/m2 and 98 +/- 23 g/m2 after 3 and 12 months respectively. Interventricular septum (11.2 to 9.8 and 9.0 mm) and posterior wall thickness (10.4 to 9.6 and 8.8 mm) also decreased significantly, whereas left ventricular internal dimensions were unchanged. Total peripheral resistance decreased significantly after initiation of treatment, but cardiac output did not change. Plasma volume, plasma renin activity and plasma noradrenaltine levels were unchanged by terazosin. Thus, terazosin monotherapy reversed LVH associated with decreased peripheral resistance. It is suggested that the reversal of LVH by terazosin is mainly due to the reduction in ventricular afterload and that humoral factors are not involved in its mechanism.