The effect on myocardial function and structure of long-term administration of quinapril (10-20 mg daily), an angiotensin-converting-enzyme (ACE) inhibitor, was investigated in 24 patients (18 men, 6 women; mean age 48 [20-65] years) with chronic isolated asymptomatic aortic or mitral regurgitation. Coronary heart disease had been excluded angiographically. After one year's treatment the regurgitation fraction, compared with the pretreatment value, had decreased by 27% in those patients with aortic regurgitation (AR) and 42% in those with mitral regurgitation (MR) (P = 0.0001). The mean left ventricular enddiastolic volume was reduced from 150 +/- 33 to 128 +/- 30 ml/m2 in patients with AR, from 146 +/- 26 to 109 +/- 24 ml/m2 in those with MR (P = 0.0001). The mean endsystolic volume fell from 55 +/- 27 to 44 +/- 28 ml/m2 in patients with AR and from 63 +/- 43 to 47 +/- 29 ml/m2 in those with MR (P = 0.002). The mean left ventricular ejection fraction at rest and on exercise rose slightly in patients with AR, remaining unchanged in those with MR. The left ventricular mass, as measured by echocardiography, was reduced by 35% in patients with AR, and the left ventricular hypertrophy, demonstrated in all patients, regressed. In patients with MR the left ventricular mass decreased by 15% and septal thickness became normal (borderline hypertrophy). These data indicate that, after one year's treatment with quinapril, left ventricular dilatation, mass and hypertrophy regressed and left ventricular function improved in patients with AR or MR.