Twenty-three patients with dilated cardiomyopathy (DCM) were treated with metoprolol and their clinical courses compared with those of 26 patients untreated with β-blocking agents (non β group). Of the 23 patients treated with metoprolol, 20 (β group) were treated for 6 months or longer, while the remaining 3 patients were intolerant of the drug and suffered aggravation of heart failure. In the β group, 2 deaths occurred, while of the remaining 18 patients 4 were considered clinically improved in NYHA class 6 or 12 months later, and none suffered clinical deterioration during the follow-up period. In the non-β group, clinical improvement was found in 2 patients and deterioration of functional class in 10. Heart rate and pressure rate product were significantly decreased by I month after the treatment with metoprolol. At that time, blood pressures, systolic and diastolic left ventricular dimensions, indices of systolic function (%FS, mVcf) and exercise capacity had not changed, while cardiac output was decreased and systemic peripheral vascular resistance was significantly increased. In the β group, significant inprovements in left ventricular dimensions, systolic function and exercise tolerance were delayed and observed within 3 to 6 months during the follow-up period, while no such improvements occurred in patients of the non- β group. During the follow-up period, 2 patients of the β group and 10 patients of the non-β group died. The survival curve for patients of the β group, prepared using the Kaplan Meier's Method, was better than that for patients of the non-β group. Metoprolol was therefore found to be useful for treatment of DCM.