Background: Preincisional and postoperative Transcutaneous Electrical Nerve Stimulation (TENS) has shown an opioid sparing effect in the treatment of postoperative pain after major spine surgery. The aim of this study was to test the effect of TENS applied intraoperatively on intraoperative demand for fentanyl. Methods: Forty-eight patients scheduled for dorsal lumbar fusion were enrolled and divided into two groups by random numbers. Group A received TENS preoperatively, intra-operatively and postoperatively. Group B received this treatment pre- and postoperatively only. Fentanyl was given during surgery if systolic blood pressure and or heart rate increased 15% over baseline level. Bispectral index (BIS) was maintained between 40 and 50. The total amount of fentanyl administered intra-operatively was recorded. Physician-determined analgesics were given postoperatively to achieve a visual analogue pain score below 3. The total amount of analgesics given, postoperative nausea and vomiting, and stomachache up to the seventh day after surgery were recorded. Results: The two groups were comparable on demographic variables. The intra-operative demand for fentanyl differed significantly between A versus B. (P>0.0000001). No significant differences were detected in total amount of administered analgesics, postoperative nausea and vomiting, nor stomachache up to the seventh postoperative day. Conclusion: Intra-operative TENS reduces intra-operative demand for fentanyl during major spinal surgery. This may be considered especially important in patients who suffer from obstructive sleep apnea syndrome.