Fentanyl lacks the antiinflammatory properties of morphine. Morphine attenuates the inflammatory response through differential stimulation of μ-receptor subtypes. Patients who receive morphine during coronary artery bypass graft have been shown to experience less postoperative fever than those who receive fentanyl. Patients who receive continuous fentanyl infusions in increased room temperatures after thermal injury may be at increased risk to experience higher body temperature than those who receive morphine. The records of 28 patients with >20%TBSA burn in 30 intensive care unit rooms (13 received fentanyl and 15 received morphine or hydromorphone) and 12 trauma patients who received fentanyl in 22°C intensive care unit rooms were reviewed. Mean maximum core temperature and percentage of temperature recordings > 39°C in the first 48 hours of admission were compared between burn patients who received fentanyl, those who did not, and with trauma patients. Burn patients exposed to fentanyl experienced significantly higher temperatures (40.1 ± 0.9°C) compared with those given morphine (38.7 ± 0.8°C) and compared with trauma patients (37.5 ± 2.4°C), P < .01 and P < .001, respectively. Burn patients on fentanyl had temperatures > 39°C for a higher percentage of time (33 ± 27%) than those without fentanyl (7.2 ± 13%) and trauma patients (1 ± 2.8%), P < .01 and P < .001, respectively. No differences in other medications administered, acute physiology and chronic health evaluation II scores, or the number of infections that could account for temperature disparities between groups existed. Burn patients who receive fentanyl in 30°C rooms experience higher body temperatures and are febrile for a higher percentage of time than those receiving morphine only. Morphine has well-established antiinflammatory properties and likely attenuates the postburn inflammatory response more than fentanyl, resulting in lower body temperatures. This phenomenon needs to be further investigated in additional studies.