The prevalence of white-matter lesions on computed tomography was studied in a representative sample of 85-year-olds living in Gothenburg, Sweden. The study included a psychiatric examination, interview of a close informant, neuropsychological examination, physical examination, comprehensive laboratory tests, electrocardiogram, chest x-ray, computed tomography scan of the head, and cerebrospinal fluid analysis. The diagnoses of dementia and other mental disorders were made according to DSM-III-R criteria. The prevalence of white-matter lesions in demented subjects was 68.9%, and in nondemented, 33.8%. Their prevalence was not increased in any mental disorder other than dementia. All severities of dementia and the subtypes, Alzheimer's disease, vascular dementia, and other types of dementia, had a significantly higher prevalence of white-matter lesions than did nondemented subjects. The risk for dementia, but not its severity, increased with the severity of these lesions. A stepwise logistic regression analysis showed that both white-matter lesions and infarcts on computed tomography contributed independently to dementia. White-matter changes may be a contributing cause of dementia in the oldest old, or may represent a disease entity of its own. They are important to recognize since they may be potentially preventable, or even treatable.