This study examined the prevalence and nature of personality change in 99 patients with dementia of the Alzheimer type and multi-infarct dementia. Personality was assessed using an informant-rated inventory of the patient's personality before and after the onset of dementia, with the difference equating to a change in personality. Personality characteristics were related to the patients' age and sex, duration of illness, degree of cognitive impairment, the presence of a grasp reflex, and extrapyramidal signs. Personality change was found to be almost universal and negative in nature and was particularly associated with severity of cognitive impairment, longer duration of illness, and neurological signs. The findings reflect those from other studies and emphasize the biological basis of personality changes in dementia.