Background: the effects of long-term administration of b-blockers on left ventricular (LV) function during exercise in patients with ischemic heart disease (IHD) and idiopathic dilated cardiomyopathy (DCM) are controversial. Patients and methods: patients with stable congestive heart failure (CHF) (New York heart association wNYHAx class II and III) and ejection fraction (EF) F0.40 were randomized to metoprolol, 50 mg t.i.d. or placebo for 6 months. Patients were divided into two groups: ischemic heart disease (IHD) and idiopathic dilated cardiomyopathy (DCM). The mean EF was 0.29 in both groups and 92% were taking angiotensin-converting enzyme (ACE) inhibitors. In the IHD group, 84% had suffered a myocardial infarction (MI) and 64% had undergone revascularization at least 6 months before the study. LV volumes were measured by equilibrium radionuclide angiography. Mitral regurgitation was assessed by Doppler echocardiography. All values are changes for metoprolol subtracted by changes for placebo. Results: metoprolol improved LV function markedly both at rest and during sub-maximal exercise in both groups. The mean increase in EF was 0.069 at rest (P-0.001) and 0.078 during submaximal exercise (P-0.001). LV enddiastolic volume decreased by 22 ml at rest (Ps0.006) and by 15 ml during exercise (Ps0.006). LV end-systolic volume decreased by 23 ml both at rest (Ps0.001) and during exercise (Ps0.004). Exercise time increased by 39 s (Ps0.08). In the metoprolol group, mitral regurgitation decreased (Ps0.0026) and only one patient developed atrial fibrillation vs. eight in the placebo group (Ps0.01). Conclusion: metoprolol improves EF both at rest and during submaximal exercise and prevents LV dilatation in mild to moderate CHF due to IHD or DCM.