We report longitudinal data on a group of 29 male patients 50 years of age or older who were initially diagnosed as having idiopathic REM sleep behavior disorder (RBD) after extensive polysomnographic and neurologic evaluations. Thirty-eight percent (11/29) were eventually diagnosed as having a parkinsonian disorder (presumably Parkinson's disease) at a mean interval of 3.7 ± 1.4 (SD) years after the diagnosis of RBD, and at a mean interval of 12.7 ± 7.3 years after the onset of RBD. To date, only 7% (2/29) of patients have developed any other neurologic disorder. At the time of RBD diagnosis, data from the RBD group with eventual Parkinson's disease (n = 11) and the current idiopathic RBD group (n = 16) were indistinguishable, with two exceptions: the RBD-Parkinson's disease group had a significantly elevated hourly index of periodic limb movements of non-REM sleep and an elevated REM sleep percentage. RBD was fully or substantially controlled with nightly clonazepam treatment in 89% (24/27) of patients in both groups. Thus, RBD can be the heralding manifestation of Parkinson's disease in a substantial subgroup of older male RBD patients. However, a number of presumed Parkinson's disease patients may eventually be diagnosed with multiple system atrophy (striatonigral degeneration subtype). Our findings indicate the importance of serial neurologic evaluations after RBD is diagnosed and implicate the pedunculopontine nucleus as a likely site of pathology in combined RED-Parkinson's disease, based on experimental and theoretical considerations rather than on autopsy data.