Left ventricular hypertrophy is considered to be a major cardiovascular risk factor in hemodialysis patients. Not only high blood pressure but also humoral factors such as angiotensin II and aldosterone are thought to contribute to the increase in left ventricular mass. We examined the effects of an angiotensin converting enzyme (ACE) inhibitor, imidapril, on left ventricular mass in patients with end-stage renal diseases on maintenance hemodialysis. Thirty patients on chronic hemodialysis were randomly divided into 2 groups of 15 patients each and given placebo or 2.5 mg imidapril once daily for 6 months. Before and after the 6-month period, left ventricular mass was evaluated by echocardiography, and circulating factors of the renin-angiotensin-aldosterone system were measured. Background characteristics such as age, gender ratio, causes of renal failure, duration of hemodialysis, body mass index and pre-dialysis blood pressure were comparable between the placebo and the imidapril groups. Systolic and diastolic blood pressures were not significantly changed in either group during the study period. In the imidapril group, serum ACE was reduced (12 +/- 1 to 5 +/- 2 U/l, p < 0.01) and plasma renin activity was increased (3.3 +/- 0.8 to 8.1 +/- 3.2 ng/ml/h, p < 0.01), but plasma angiotensin II and aldosterone were not significantly changed after 6 months (13 +/- 3 to 17 +/- 3 pg/ml and 365 +/- 125 to 312 +/- 132 pg/ml, respectively). On the other hand, left ventricular mass index was significantly decreased in the imidapril group (132 +/- 10 to 109 +/- 6 g/m2, p < 0.05) but was unchanged in the placebo group (129 +/- 6 to 126 +/- 5 g/m2). These results suggest that an ACE inhibitor reduces left ventricular mass in hemodialysis patients by a mechanism that is independent of changes in blood pressure.