The effect of cibenzoline on left ventricular diastolic function was investigated in patients with hypertrophic cardiomyopathy (HCM). Before and 2 h after an oral administration of 200 mg of cibenzoline, echocardiographic, apexcardiographic and gated radionuclide angiographic studies were performed in 12 patients with hypertrophic obstructive cardiomyopathy (HOCM) and 7 with hypertrophic nonobstructive cardiomyopathy (HNCM). After administration of cibenzoline, the left ventricular pressure gradient decreased from 96+/-33 mmHg to 29+/-22 mmHg (<0.0001). Fractional shortening decreased from 53.3+/-7.5 to 45.4+/-6.2% (p=0.0008) in patients with HOCM and from 49.9+/-8.7 to 40.9+/-7.5% (p=0.0039) in patients with HNCM. On the other hand, E-wave velocity increased and A-wave velocity decreased in both groups. The time between the second heart sound and O point was shortened from 253+/-53 to 176+/-21 ms (p<0.0001) in patients with HOCM and from 245+/-54 to 185+/-44 ms (p=0.0050) in patients with HNCM. The time to peak filling rate was shortened from 248+/-79 to 190+/-40 ms (p=0.0072) in patients with HOCM and from 218+/-33 to 163+/-26 ms (p=0.0052) in patients with HNCM. These results indicate that in patients with HCM, cibenzoline suppresses left ventricular systolic function, but can markedly improve left ventricular diastolic dysfunction through its direct action.