Delirium, with occurrence rates from 14% to 56%, associated mortality rates from 10% to 65%, and excess annual health care expenditures from $1 to $2 billion, poses a common and serious problem for hospitalized elderly patients. Delirium is often unrecognized or misdiagnosed by physicians caring for elderly patients. Cognitive testing is rarely done as part of the admission evaluation of elderly hospitalized patients. Specific diagnosis has been difficult, since diagnostic criteria and instruments are still being developed. The etiology of delirium is complex and multifactorial, and both predisposing (host vulnerability) and precipitating factors must be considered. The recommended approach to the evaluation of delirium is empiric, in the absence of objective efficacy data. The cornerstone of evaluation includes a careful history, physical examination, and review of the medication list--since medications are the most common reversible cause of delirium. Research is needed to establish a cost-effective approach and to clarify the role of further testing, such as cerebrospinal fluid examination, brain imaging, and electroencephalography. This article is intended to heighten the awareness of clinicians as well as to stimulate research to address this important, neglected problem for elderly hospitalized patients.