UNLABELLED: In chronic heart failure, elevated plasma norepinephrine (NE) levels and a disparity between the neuronal release and the effective reuptake of NE lead to an increased concentration of NE in the presynaptic cleft, causing a downregulation of the myocardial beta-adrenoceptors. The clinical and prognostic effectiveness of beta-blocker therapy has been shown in patients with chronic heart failure in several large trials. The purpose of this study was to investigate the effect of long-term beta-blocker therapy on the cardiac adrenergic nervous system as assessed by the myocardial uptake of 123I-metaiodobenzylguanidine (MIBG), an analog of NE, in idiopathic dilated cardiomyopathy (IDC). METHODS: In 10 patients with IDC and stable chronic heart failure the myocardial MIBG uptake was measured at baseline and at 1 y (median, 11.5 mo) after treatment with beta-blockers (metoprolol, n = 5; bisoprolol, n = 1; and carvedilol, n = 4) in addition to standard medication. In parallel with the changes in MIBG uptake, the New York Heart Association functional class, the left ventricular ejection fraction (LVEF), and the left ventricular end-diastolic diameter (LVEDD) were documented before and after 1 y of therapy with beta-blockers. RESULTS: During the 1-y follow-up, a significant increase in myocardial 123I-MIBG uptake (P = 0.005) in parallel with an improved LVEF (P = 0.005) and a reduced LVEDD (P = 0.019) was found. A trend toward an improvement of the New York Heart Association functional class under the beta-blocker therapy (P = 0.139) was also found. CONCLUSION: Assessment of the myocardial 123I-MIBG uptake is a useful noninvasive tool for evaluating changes in cardiac sympathetic nerve activity under medical therapy. Long-term treatment with beta-blockers in IDC causes a recovery of the cardiac adrenergic nervous system concomitantly with a clinical and hemodynamic improvement.