A follow-up of 35 patients first diagnosed as having presenile dementia at York Clinic is described. In only 15 cases did progressive deterioration confirm the diagnosis. A retrospective case note study of all the patients is described comparing the clinical features and results of special investigations of those patients in whom the diagnosis was confirmed in this way and those in whom it was not. The results of this study are used to illuminate some of the special difficulties in the early diagnosis of presenile dementia.