AbstractForty‐three patients with ‘late paraphrenia’ were subjected to detailed clinical and psychological assessment and computed tomography (CT). The paraphrenics had significantly larger lateral cerebral ventricles and greater cognitive deficits than a group of 40 age‐matched normal volunteers. Group comparisons within the patients, between those with particularly large ventricular brain ratio (VBR) (equal or more than two standard deviations above the control mean) and the rest of the group, showed that the extent of ventricular enlargement had no direct bearing on the clinical presentation of the paraphrenic syndrome. The results suggest that ventricular enlargement may have preceded the onset of overt symptoms by a period of years and probably served as a non‐specific risk factor for the development of the disorder. The unimodal distribution of VBR values suggests that paraphrenia, at least at a brain structural level, is a unitary condition. It also indicates the presence of subcortical pathology which might be in part responsible for the cognitive dysfunction elicited.