Delirium, an acute confusional state, is an organic brain syndrome that manifests deficits in attention, irrelevant or rambling speech, and other cognitive deficits. Its symptoms often fluctuate over the course of the day, and patients may be hyperactive--for example, restless and screaming--or hypoactive--for example, quiet, inactive, and stuporous. Occurring in approximately 20% of hospitalized elderly patients, delirium is the most common psychiatric syndrome in acutely ill general medical and surgical patients. Fifteen to 30% of delirious patients expire, and others are prone to a variety of complications: falls, pressure ulcers, oversedation, dehydration, and others. Almost any acute illness can cause delirium in the elderly, but the most common offenders are acute infections and drugs. Many patients have a pre-existing dementia. The first step in arriving at a correct diagnosis is to distinguish delirium from other psychiatric syndromes that can cause confusion, such as dementia, depression, schizophrenia, and mania. Once delirium is established, a comprehensive general examination and a mental status examination is required. Routine laboratory and radiologic tests are directed at the common metabolic and infectious disorders that precipitate delirium. Treatment is directed at the underlying acute illness. In all patients, it is important (1) to treat the underlying acute illness, (2) to provide appropriate fluid and electrolytes, (3) to discontinue any unnecessary drugs, and (4) to allay the patient's fear and agitation through the use of simple, repetitive instructions, orientation cues, and by limiting the use of physical restraints. If psychotropic medications are needed to treat psychotic symptoms, to prevent patients from harming themselves or others, or to facilitate necessary diagnostic and therapeutic interventions, then haloperidol is the drug of choice in most instances. Drugs with anticholinergic properties should be avoided.