Background: Prevention of hypothermia in patients undergoing major posterior approach spinal surgery can be difficult, as a large body surface is exposed to the cold environment of the operating theatre. We compared the efficacy of a new under-body forced-air warming blanket with that of a resistive heating blanket in preventing hypothermia. Methods: Sixty patients undergoing major posterior approach spinal surgery lasting for more than 2 hours were randomly assigned to warming with a full under-body forced-air warming blanket or three segments of resistive heating blankets, both set at 42°C. The ambient temperature was kept near 20°C. Nasopharyngeal, rectal and axillary temperatures were measured at regular intervals. Changes in core temperature (average of nasopharyngeal and rectal) over time were compared by the independent t-test. Results: The characteristics of the patients were comparable. The baseline core temperature was 36.36 ±0.38°C in the forced-air group and 36.27 ± 0.46°C in the resistive heating group. During the first hour, the core temperature decreased similarly from baseline in both groups. From 100 minutes after induction until the end of the surgery, core temperature rose in both groups. At the end of surgery, the core temperature was increased by 0.08 ± 0.09°C from baseline in the forced-air group but decreased by 0.40 ±0.04°C from baseline in the resistive heating group. The difference in the change of the core temperature, at the end of the surgery, between the two groups is statistically significant (P<0.05). Conclusion: We demonstrated that the new under-body forced-air warming blanket is superior to the resistive heating blanket in preventing hypothermia in patients undergoing major posterior approach spinal surgery.