The prevalence, risk factors, and outcomes of delirium were studied in 229 elderly patients. Fifty patients (22%) met criteria for delirium; nondelirious elderly constituted the control group. Abnormal sodium levels, illness severity, dementia, fever or hypothermia, psychoactive drug use, and azotemia were associated with risk of delirium. Patients with three or more risk factors had a 60% rate of delirium. Delirious patients stayed 12.1 days in the hospital vs 7.2 days for controls and were more likely to die (8% vs 1%) or be institutionalized (16% vs 3%). Illness severity predicted 6-month mortality, but the effect of delirium was not significant. Delirium occurs commonly in hospitalized elderly, is associated with chronic and acute problems, and identifies elderly at risk for death, longer hospitalization, and institutionalization. The increased mortality associated with delirium appears to be explained by greater severity of illness.