Delirium is a common mental disorder in the elderly with old age being a major risk factor for delirium. Another major risk factor is dementia. The aim of the present study was to identify differences in occurrence of episodes of delirium between the most common dementia diagnoses and the possible importance of age, gender, severity and duration of dementia for the development of delirium. Included in the study were 175 consecutive patients with probable Alzheimer's disease, vascular dementia (VAD) or frontotemporal dementia (FTD) who were admitted to a neuropsychiatric diagnostic unit. There were no significant differences in sex distribution or duration of dementia between the delirious and non delirious patients. The rate of delirium was higher in late onset Alzheimer's disease (LAD) than in early onset Alzheimer's disease (EAD) and FTD. It was also higher in VAD than in EAD. The differences in occurrence of delirium between the diagnostic groups in this sample could not be explained by differences in age. It seems that delirium is more common in brain disorders such as LAD and VAD in which the damage to the brain is more widespread. In the two brain disorders that are predominantly cortical, EAD and FTD, the occurrence of delirium was comparatively low.