BACKGROUND: General anesthesia often produces some degree of hypothermia and hypothermia causes much more blood loss during surgery than normothermia. Electrically heated humidifiers (EHHs) have been used for patients under general anesthesia and in the intensive care unit. However, the benefits of the EHH have not been widely reported in the literature. METHODS: Patients scheduled for posterior lumbar spine fusion, were randomly assigned to a mechanically ventilated with EHH circuit group or to a conventional respiratory circuit group. Their tympanic membrane temperature was monitored every 30 min after induction up to 180 min, and perioperative blood losses, transfusion requirements during surgery, and other complications were noted. RESULTS: Patients in the control group (n = 40) showed a lower mean body temperature at all times than immediately after induction, while the EHH group (n = 40) showed a lower body temperature from 60 minute after induction comparing to the initial temperature. Furthermore, patients in the EHH group had a higher mean body temperature than patients in the control group during surgery (35.9 +/- 0.4 vs 35.4 +/- 0.5, P < 0.001). Mean intraoperative blood loss (9.75 +/- 5.4 vs 7.48 +/- 3.9, P = 0.035) and transfusion requirements (57.5% vs 25%, P = 0.006) were significantly less in the EHH group, but postoperative blood loss, duration of hospitalization, and other complications were not significantly different in the two study groups. CONCLUSIONS: The use of an electrically heated humidifier did not prevent a body temperature drop under general anesthesia. However, it helped maintain body temperature and was associated less blood loss and transfusion requirement during surgery.