Although decrease in myocardial iodine-123 metaiodobenzylguanidine ((123)I-MIBG) radioactivity has been reported in patients with rapid eye movement (REM) sleep behavior disorder (RBD), its pathophysiology has not been thoroughly disclosed. We report two RBD patients with differing clinical progression, in whom myocardial (123)I-MIBG scintigraphy was performed. One 69-year-old patient had more than a 20-year history of idiopathic RBD and showed a decrease in myocardial (123)I-MIBG radioactivity. The other 69-year-old patient started to manifest nocturnal behaviors at age 62, then mild parkinsonism at age 68, and showed a similar decrease in myocardial (123)I-MIBG radioactivity both before and after the onset of parkinsonism. These cases suggest that RBD could develop in diverse patterns of clinical progression even if signs of underlying Lewy body pathology are uniformly indicated.