In a prospective study of late-life onset psychosis, five of the first 27 patients studied had extensive white-matter lesions demonstrated by MRI and/or CT. None of 60 age-matched psychiatrically healthy controls demonstrated such lesions. All five patients had a mild dementia and a frontal behavioural syndrome. In addition, every patient performed poorly on neuropsychological tests of frontal function. Dysfunction of the frontal cortex associated with white-matter lesions appears to contribute to the clinical picture of some cases of late-life psychosis.