The purpose of this study was to describe the natural history, identify risk factors, and determine outcomes for the development of postoperative delirium in the elderly. Postoperative delirium is a common and deleterious complication in geriatric patients. Subjects older than 50 years scheduled for an operation requiring a postoperative intensive care unit admission were recruited. After preoperative informed written consent, enrolled subjects had baseline cognitive and functional assessments. Postoperatively, subjects were assessed daily for delirium using the confusion assessment method-intensive care unit. Patients were also followed for outcomes. During the study period, 144 patients were enrolled before major abdominal (40%), thoracic (53%), or vascular (7%) operations. The overall incidence of delirium was 44% (64/144). The average time to onset of delirium was 2.1 +/- 0.9 days and the mean duration of delirium was 4.0 +/- 5.1 days. Several preoperative variables were associated with an increased risk of delirium including older age (P < 0.001), hypoalbuminemia (P < 0.001), impaired functional status (P < 0.001), pre-existing dementia (P < 0.001), and pre-existing comorbidities (P < 0.001). In a multivariable logistic regression model, pre-existing dementia remains the strongest risk factor for the development of postoperative delirium. Worse outcomes, including increased length of stay (P < 0.001), postdischarge institutionalization (P < 0.001), and 6 month mortality (P = 0.001), occurred in subjects who developed delirium. In the current study, delirium occurred in 44% of elderly patients after a major operation. Pre-existing cognitive dysfunction was the strongest predictor of the development of postoperative delirium. Outcomes, including an increased rate of 6 month mortality, were worse in patients who developed postoperative delirium.