To evaluate sympathetic dysfunction of the heart, one of the factors responsible for heart failure in patients with dilated cardiomyopathy (DCM), we performed 123I-metaiodobenzylguanidine (123I-MIBG) myocardial single photon emission tomography (SPET) in 22 cases of DCM. There were significant relationships between severity scores and defect scores for the early and delayed images. When the data acquired by 123I-MIBG myocardial SPET were compared with left ventricular function, the extent score (r = -0.754, P < 0.01) and severity score (r = 0.693, P < 0.01) for the early images were both significantly correlated with left ventricular ejection fraction (LVEF). For the delayed images also, a good correlation was obtained between LVEF and extent score (r = -0.704, P < 0.01) and between LVEF and severity score (r = -0.801, P < 0.01). the washout rate for the entire left ventricle, calculated from the early and delayed images, also correlated with LVEF (r = -0.643, P < 0.01). Since the 123I-MIBG defect on the early images was shown to be significantly related to left ventricular function, a decrease in neuronal uptake at sympathetic nerve endings appears in the main to account for the 123I-MIBG defect on the early images. The regional washout rate was increased compared to that in the control group (P < 0.01). It was particularly high in the inferior wall, suggesting that acceleration of turnover and poor retention in the area of the 123I-MIBG defect on the early images and then the entire left ventricle was strongly associated with the 123I-MIBG defect on the delayed images. Our results suggest that 123I-MIBG myocardial SPET may be useful in determining the severity of DCM.