Rapid eye movement (REM) sleep behavior disorder (RBD) is characterized by loss of muscular atonia and prominent motor behaviors during REM sleep, associated with excessive motor activity while dreaming. Behaviors can cause injuries to the patient or sleeping partner. RBD is associated with neurodegenerative diseases, such as multiple system atrophy, Parkinson's disease, dementia with Lewy bodies, and progressive supranuclear palsy. In many cases, the symptoms of RBD occur before other symptoms of these neurodegenerative disorders by several years. RBD was noted for the first time after the patient developed cancer of the prostate. The fact that PCa develops one year after the onset of RBD is of significance. Could RBD be a forerunner for PCa, or could a PCa that has not yet clinically surfaced have given rise to RBD? Some diseases associated with voltage gated potassium (K) channel antibody also had RBD. The possible contribution of voltage gated K channel activity in metastatic process was emphasized in PCa model in rats. Apparently, being able to explain the development mechanism of RBD by PCa and K channel antibody might be a possible viewpoint. Although our hypotheses can be explained by voltage gated K channel activity, yet there are still numerous ambiguities. This case brings to focus several unanswered questions related to the pathogenesis of RBD.