In an attempt to evaluate EEG changes associated with dementia in Parkinson's disease (PD), we performed frequency analysis in three groups of 10 subjects each; two with PD, and one normal control group. The PD patient groups were matched for age, sex, severity and duration of disease, but were discordant for the existence of dementia. Normals were age- and sex-matched healthy volunteers. The relative alpha amplitude was significantly decreased in the demented PD patients, unrelated to motor disability. There was a nonsignificant but consistent trend of increased amplitude in the delta and theta range in the demented PD patients as compared to nondemented PD subjects and normal controls, as well as increased amplitude in the theta and delta range with more severe motor disability in the nondemented PD patients.