A randomized and prospective study was designed to prove the efficacy of angiotensin II receptor blockers (ARB) in the amelioration of myocardial impairment in hypertrophic nonobstructive cardiomyopathy (HNCM). Nineteen consecutive patients with HNCM were randomly assigned to two groups and then underwent cine magnetic resonance evaluation of left ventricular mass (LVM) twice just before and after one year of observation. In the ARB group, 50 mg of losartan potassium was administered once daily during the observation period. The ratio of LVM after the observation period over that before the period was blindly compared between the two groups to estimate morphologically the ameliorative effect of ARB. In the ARB group, LVM was 203 +/- 47 cm(3) before the treatment period and 190 +/- 55 cm(3) after the period and the ratio of the final LVM over the initial LVM was 0.93 +/- 0.10. In the non-ARB group the initial and final LVM values were 177 +/- 48 cm(3) and 179 +/- 45 cm(3), and the ratio of the final LVM over the initial LVM was 1.02 +/- 0.07. The ratio of the final LVM over the initial LVM in the ARB group was significantly smaller (P = 0.03) than that in the non-ARB group. The smaller ratio in the ARB group strongly indicates that ARB ameliorated the natural course of HNCM during the one year observation period. Thus, this is the first demonstration of the therapeutic efficacy of ARB in human HNCM.