Delirium in a patient with preexisting dementia is a common problem that may have serious complications and poor prognostic implications. The purpose of this paper was to conduct a systematic review of the medical literature on delirium superimposed on dementia, specifically to review studies on prevalence, associated features, outcomes, and management. Areas of controversy and gaps in our knowledge of this problem are highlighted. Finally, an agenda for future research is proposed. Fourteen studies were reviewed, including seven prospective studies, three retrospective studies, two cross-sectional studies, and two clinical trials. For the review of the literature on delirium superimposed on dementia, we searched MEDLINE from January 1966 through February 2002 for research studies with primary sources of data. Selection criteria for inclusion of articles in this study were inclusion of data on subjects with delirium superimposed on dementia, inclusion of a validated operational definition/measures of dementia and delirium, actual data on persons with delirium and dementia reported in the paper, and reporting of primary data. MEDLINE was searched using the following key search terms: delirium, acute confusion, cognitive impairment, Alzheimer's disease, dementia, delirium superimposed on dementia, and elderly. The prevalence of delirium superimposed on dementia ranged from 22% to 89% of hospitalized and community populations aged 65 and older with dementia. To date, only one reported study systematically identified associated factors and interventions for delirium superimposed on dementia, but several studies examining outcomes have found that adverse events are associated with delirium in persons with dementia, including accelerated and long-term cognitive and functional decline, need for institutionalization, rehospitalization, and increased mortality. This paper highlights the dearth of research on delirium superimposed on dementia and stresses the importance of early recognition and prevention of delirium in persons with dementia.