Circadian disturbances in the autonomic nervous system, sleep and the endocrine system are common after major surgery. We examined whether the circadian regulation of core temperature was disturbed in 10 patients undergoing major abdominal operations. Patients were monitored on the day before and on the second postoperative day with intravesical temperature. Circadian variation was present preoperatively (P < 0.001), whereas circadian rhythm in core temperature was abolished on the second postoperative night (P = 0.73). Postoperative circadian disturbances may be related to morbidity and convalescence, and further studies should therefore focus on mechanisms and prevention.