Idiopathic REM sleep behavior disorder (RBD) predicts Parkinson's disease (PD) and dementia. However, the nature of the disease that emerges from RBD has not been fully characterized. Since 2004, we have been conducting a prospective study of idiopathic RBD patients, providing an opportunity to directly observe patients as they transitioned to a defined neurodegenerative syndrome. Patients with idiopathic RBD underwent an extensive annual evaluation of motor function, olfaction, color vision, autonomic function, cognition and psychiatric symptoms. Neurodegenerative disease was defined according to standard criteria. We compared these measures in patients who had developed PD to those with dementia, all within the first year of developing disease. Of 67 patients, 6 developed PD and eleven developed dementia. Except for cognitive functioning, all tests of olfaction, color vision, autonomic function, depression, and quantitative measures of motor speed were similar in patients with PD and dementia. Of dementia patients, seven met criteria for probable Lewy body dementia (LBD) and four for Alzheimer's disease (or, possible LBD). In all probable LBD cases, the diagnosis was made because of parkinsonism, with no patient experiencing hallucinations or fluctuations. Patients with "Alzheimer's disease" seemed to have LBD, as they demonstrated typical LBD cognitive profiles on neuropsychological testing and were indistinguishable from LBD patients in ancillary measures. Therefore, among RBD patients with new-onset LBD, hallucinations or fluctuations are absent, suggesting that RBD is a reliable early sign of LBD. The indistinguishability of dementia and PD in all ancillary measures suggests a single unitary "RBD-then-neurodegeneration" process, the clinical presentation of which depends upon selective neuronal vulnerability.