We performed [123I]-meta-iodobenzylguanidine (MIBG) myocardial scintigraphy in 180 patients with PD, 24 patients with DLB and 73 patients with the other related diseases. The heart-to-mediastinum (H/M) ratio in PD and DLB was significantly low compared to that in MSA, PSP, CBD and AD. The H/M ratio tended to decrease with the disease progression in PD. These findings suggest that MIBG myocardial scintigraphy could be a marker for differential diagnosis of PD and DLB. We immunohistochemically examined heart tissues from patients with pathologically confirmed PD, DLB and the other related diseases using monoclonal antibody against tyrosine hydroxylase (TH). The number of TH-immunoreactive nerve fibers in the epicardium was moderate or abundant in most patients with MSA, PSP, CBD and AD as well as in the control subjects. Contrarily, the number of TH-immunoreactive nerve fibers in the epicardium had almost completely disappeared in nearly all the patients with PD and DLB. These findings suggest that postganglionic cardiac sympathetic nerve denervation occurs in PD and DLB but not in MSA, PSP, CBD and AD, which accounts for the decreased cardiac uptake of MIBG in PD and DLB, but not in the other diseases.