Postoperative delirium is common in geriatric patients. Few studies have examined events in the postoperative period that may contribute to the occurrence of postoperative delirium. We hypothesized that postoperative delirium is related to postoperative pain and/or pain management strategy. Patients aged > or =65 years who were scheduled for major noncardiac surgery were studied. A structured interview was conducted preoperatively and for the first 3 postoperative days to determine the presence of delirium using the Confusion Assessment Method. The method of postoperative pain management, as well as pre- and postoperative medications for the first 3 days, was collected. Pre- and postoperative pain at rest and with movement was recorded using the Visual Analog Scale. Three hundred thirty-three patients, with a mean age of 74 +/- 6 years, were studied. After surgery, 46% of patients developed postoperative delirium. By multivariate logistic regression, age (odds ratio [OR], 2.5; 95% confidence interval [CI] 1.5 to 4.2), moderate (OR, 2.2; 95% CI 1.2 to 4.0) and severe (OR, 3.7; 95% CI 1.5 to 9.0) preoperative resting pain, and increase in level of pain from baseline to postoperative day one (OR, 1.1; 95% CI 1.01 to 1.2) were independently associated with a greater risk for the development of postoperative delirium. In contrast, patients who used oral opioid analgesics as their sole means of postoperative pain control were at decreased risk of developing delirium in comparison with those who used opioid analgesics via IV patient-controlled analgesia technique (OR, 0.4; 95% CI 0.2 to 0.7). These results validate our hypothesis that pain and pain management strategies are important factors related to the development of postoperative delirium in elderly patients.