The purpose of this study was to determine whether dementia with Lewy bodies with and without probable rapid eye movement sleep behavior disorder differ clinically or pathologically. Patients with dementia with Lewy bodies (DLB) with probable rapid eye movement sleep behavior sleep disorder (n = 71) were compared with those without it (n = 19) on demographics, clinical variables (core features of dementia with Lewy bodies, dementia duration, rate of cognitive/motor changes), and pathologic indices (Lewy body distribution, neuritic plaque score, Braak neurofibrillary tangle stage). Individuals with probable rapid eye movement sleep behavior disorder were predominantly male (82% vs 47%) and had a shorter duration of dementia (mean, 8 vs 10 years), earlier onset of parkinsonism (mean, 2 vs 5 years), and earlier onset of visual hallucinations (mean, 3 vs 6 years). These patients also had a lower Braak neurofibrillary tangle stage (stage IV vs stage VI) and lower neuritic plaque scores (18% vs 85% frequency), but no difference in Lewy body distribution. When probable rapid eye movement sleep behavior disorder developed early (at or before dementia onset), the onset of parkinsonism and hallucinations was earlier and Braak neurofibrillary tangle stage was lower compared with those who developed the sleep disorder after dementia onset. Women with autopsy-confirmed DLB without a history of dream enactment behavior during sleep had a later onset of hallucinations and parkinsonism and a higher Braak NFT stage. Probable rapid eye movement sleep behavior disorder is associated with distinct clinical and pathologic characteristics of dementia with Lewy bodies.