Data from 101 consecutively admitted intensive care unit (ICU) patients were examined to determine whether oxidative metabolic stress existed within the 48 hours before delirium onset. The occurrence of pneumonia and sepsis at any time during hospitalization was also recorded. Delirium was defined retrospectively with the Confusion Assessment Method (CAM). Older patients were found to develop delirium more frequently than younger patients. There were no differences in illness severity (APACHE II) between those who developed delirium and those who did not. Three measures of oxygenation (hemoglobin, hematocrit, pulse oximetry) were worse in the patients who later developed delirium. Two measures of oxidative stress (sepsis, pneumonia) occurred more frequently among those diagnosed with delirium. Hence, patients with indicators of oxidative dysfunction developed delirium more frequently, and this was not linked to illness severity.