Iodine-123 (I-123) meta-iodobenzylguanidine (MIBG) imaging was performed in 31 patients. Three patients were without cardiac disease and 28 had idiopathic dilated cardiomyopathy with various degrees of left ventricular dysfunction. The qualitatively assessed myocardial I-123 MIBG scintigrams and the myocardial versus mediastinal I-123 MIBG uptake ratio were related to I-123 MIBG activity and norepinephrine concentration determined from endomyocardial biopsy samples taken from the right side of the interventricular septum. Scintigrams and the MIBG uptake ratio were also related to plasma catecholamine concentrations, left ventricular ejection fraction and New York Heart Association functional class. Patients with distinct myocardial I-123 MIBG uptake (score 1) had a normal ejection fraction (58 +/- 16%). Patients with diffusely reduced uptake or scintigraphic defects (score 2) had a significantly lower ejection fraction (38 +/- 9%, p less than 0.05), whereas patients with shadowy or no visible myocardial uptake (score 3) had the lowest ejection fraction (23 +/- 6%, p less than 0.002 versus patients with score 2). The scintigraphically determined I-123 MIBG activity in the septal region correlated significantly with I-123 MIBG activity from the endomyocardial biopsy samples (r = 0.78, p less than 0.001, n = 9). The myocardial versus mediastinal I-123 MIBG activity ratio was significantly related to myocardial norepinephrine concentration (r = 0.63, n = 28) and to left ventricular ejection fraction (r = 0.74, n = 31). These data suggest that myocardial I-123 MIBG scintigraphy is a useful noninvasive method for the assessment of myocardial adrenergic nervous system disintegrity in patients with idiopathic dilated cardiomyopathy.