There have been over two decades of research on delirium in hospitalized older adults, yet little is known about the etiologic mechanisms and risk factors for delirium in persons with dementia. As increasing numbers of persons are diagnosed at earlier stages of dementia, attention to preventable and treatable problems like delirium are crucial. This paper summarizes work by Fick and others on delirium superimposed on dementia (DSD) and explores the implications for practice and future research. The few studies of delirium that have not excluded persons with dementia demonstrate a high prevalence (22-89%) of DSD in both community and hospital populations. In one study of DSD, persons were treated for higher rates of cerebrovascular disease, urinary tract infection, dehydration, and pneumonia and had higher total healthcare costs, more emergency room visits, and more short-stay nursing home visits than did persons with dementia alone and delirium alone. Prevention and early detection of delirium may avoid costly and prolonged hospitalization or institutionalization in a rapidly expanding population of older persons with dementia.