Rapid eye movement sleep behavior disorder may herald several neurodegenerative disorders associated with parkinsonism, including Parkinson's disease. A 72-year-old man with a 17-year history of rapid eye movement sleep behavior disorder confirmed by polysomnography developed a progressive dementia that met operational clinical criteria for diffuse Lewy body disease. The differential diagnosis of progressive neurodegenerative disorders heralding as rapid eye movement sleep behavior disorder should now include diffuse Lewy body disease.