BACKGROUND AND PURPOSE: Temperature is a well-known modulator of experimental cerebral injury. We hypothesized that hyperthermia would be associated with a worsened cognitive outcome after coronary artery bypass graft surgery (CABG). METHODS: Three hundred consenting patients undergoing cardiopulmonary bypass for CABG had hourly postoperative temperatures recorded. The degree of postoperative hyperthermia was determined by using the maximum temperature within the first 24 hours as well as by calculating the area under the curve for temperatures >37 degrees C. Patients underwent a battery of cognitive testing both before surgery and 6 weeks after surgery. By use of factor analysis, 4 cognitive domains (scores) were identified, and the mean of the 4 scores was used to calculate the cognitive index (CI). Cognitive change was calculated as the 6-week CI minus the baseline CI. Multivariable linear regression (controlling for age, baseline cognitive function, and temperature during cardiopulmonary bypass) was used to compare postoperative hyperthermia with the postoperative cognitive change. RESULTS: The maximum temperature within the first 24 hours after CABG ranged from 37.2 degrees C to 39.3 degrees C. There was no relationship between area under the curve for temperatures >37 degrees C and cognitive dysfunction (P=0.45). However, the maximum postoperative temperature was associated with a greater amount of cognitive dysfunction at 6 weeks (P=0.05). CONCLUSIONS: This is the first report relating postoperative hyperthermia to cognitive dysfunction after cardiac surgery. Whether the hyperthermia caused the worsened outcome or whether processes that resulted in the worsened cognitive outcome also produced hyperthermia requires further investigation. In addition, interventions to avoid postoperative hyperthermia may be warranted to improve cerebral outcome after cardiac surgery.