Rapid eye movement (REM) sleep behavior disorder (RBD) involves complex behavior and a loss of muscle atonia occurring during REM sleep. Half of these patients with RBD have an underlying neurologic disorder including dementia, olivopontocerebellar atrophy, subarachnoid hemorrhage, and cerebrovascular disease. Clonazepam is the drug of choice for RBD. RBD has been rarely reported to precede the onset of Parkinson's disease (PD). Three patients are presented here whose RBD preceded the onset of PD by several years, and both the symptoms of PD and RBD improved with levodopa treatment. It is postulated that levodopa ameliorates RBD by suppressing REM sleep, and it remains to be seen whether levodopa can be an alternative to clonazepam in idiopathic RBD without PD.