SUMMARYThis is a report of a clinical case presentation from the Neuropsychiatry Conference held at Fulbourn Hospital, Cambridge on March 13, 1992. The conference, chaired by Dr Nigel Hymas, was asked to comment on the diagnosis of a case first referred during September 1990. A 60-year-old man with no previous history of psychiatric illness presented with persecutory ideation, delusions and hallucinations in the absence of cognitive impairment. At the time, the initial tentative psychiatric diagnosis was one of late paraphrenia, but it later became clear that the patient was dementing. Were the paraphrenia and dementia related? If so, how? What was the nature of the dementing illness? Could life-long personality abnormalities have been the earliest expression of cerebral abnormalities predisposing him to his current illness? Incompatible evidence from clinical features, functional brain scanning and comprehensive neuropsychological testing demonstrates the inadequacy of current diagnostic criteria.