We evaluated whether myocardial b-adrenergic receptor (b-AR) density, as determined by 11 C-CGP12177 PET, could predict improvement of cardiac function by b-blocker carvedilol treatment in patients with idiopathic dilated cardiomyopathy (IDC). Methods: Ten patients with IDC (left ventricular ejection fraction [LVEF] , 45%) were studied. Myocardial b-AR density was estimated using 11 C-CGP12177 PET before treatment with carvedilol. Changes of LVEF in response to dobutamine infusion (DLVEF-dobutamine) were also measured by echocardiography. Changes of LVEF (DLVEF-carvedilol) were evaluated after 20 mo of carvedilol treatment. Results: Baseline myocardial b-AR density significantly correlated with DLVEF-carvedilol (r 5 20.88, P , 0.001). In contrast, DLVEF-dobutamine did not correlate with DLVEF-carvedilol (P 5 0.65). Myocardial b-AR density was the significant multivariate independent predictor of DLVEF-carvedilol (b 5 20.88, P , 0.001) among univariate predictors, including functional class (r 5 0.76, P , 0.05), plasma norepinephrine (r 5 0.85, P , 0.01), LVEF (r 5 20.64, P , 0.05), and age as confounding factors. Furthermore, myocardial b-AR density was significantly correlated with plasma norepinephrine (r 5 20.79, P , 0.01) and LVEF (r 5 0.70, P , 0.05). Conclusion: Myocardial b-AR density is more tightly related to improvement of LVEFcarvedilol than is cardiac contractile reserve in patients with IDC. Patients with decreased myocardial b-AR have higher resting adrenergic drive, as reflected by plasma norepinephrine, and may receive greater benefit from being treated by antiadrenergic drugs.