Propofol is widely used for general anesthesia because of its rapid onset and recovery. We had four cases that had higher body temperatures toward the end of anesthesia. The etiology of the hyperthermia is discussed.Four patients (three male, one female, 22-26 yr of age, weighing 53-57 kg) with facial deformities were anesthetized with propofol infusion (3-10 mg x kg(-1) x hr(-1)) and fentanyl (400-1,300 microg) without nitrous oxide, immobilized with vecuronium bromide (18-37 mg) or pancuronium bromide (31 mg). In order to reduce blood loss and improve the surgical view, tri-nitro-glycerin (TNG) was used in all cases. Osteotomy of maxilla and mandible or sagittal split ramus osteotomy of mandible was successfully performed. Although their body temperatures were normal preoperatively and stable during the operation, toward the end of anesthesia (one hour), they increased to over 38 degrees C. The room temperature was decreased and the water blanket on the operating table was also decreased. In addition, cool crystalloid solution was infused. Body temperature returned to normal in the ward and no complications due to the high temperature were seen postoperatively. It is thought that lighter anesthesia was masked by continuous infusion of propofol and TNG-induced hypotension and that benign hyperthermia occurred toward the end of anesthesia.Propofol reduces blood pressure, which suggests deep anesthesia. However, care must be taken to maintain the optimum depth of anesthesia during propofol anesthesia, especially when deliberate hypotensive anesthesia is induced.