Purpose: To evaluate if a Full Access Underbody (FAU) blanket used preoperatively and intraoperatively in patients undergoing major spinal surgery prevents hypothermia compared with current practice and to explore patients' experiences of comfort. Design: A nonrandomized controlled trial. Methods: Sixty patients were included, 30 in each group. Temperature was assessed on arrival, after connecting to the bladder catheter, and at the start and end of surgery. In the FAU group, comfort was evaluated at arrival and after 10 minutes of prewarming. Findings: The incidence of hypothermia at the start of surgery was significantly lower (relative risk [95% confidence interval], 0.28 [0.13 to 0.59]). Before prewarming, 77% felt comfortable, 20% cold, and 3% hot. After prewarming 60% felt comfortable, 37% hot, and 3% very hot. Conclusions: Patients using the FAU blanket had a 72% lower incidence of hypothermia at the start of the operation. Attention to thermal comfort during surgery is important.