BACKGROUND: Postoperative low back pain is common. Some studies have indicated that the use of a lumbosacral wedge is effective in reducing the incidence of postoperative low back pain. This study aimed to evaluate the effect of knee flexion with the patient in the supine position and under general anesthesia for reducing postoperative low back pain. METHODS: 261 patients who were undergoing surgery under general anesthesia were randomized to the supine position (Group I) or the supine position with both knees in flexion (Group II). The incidence of low back pain and the pain score (VAS) were measured at the following time points: postoperatively 6 hours, 1 day, 2 days, 3 days and 4 days. RESULTS: The incidence of low back pain in group II was significantly lower than that in Group I at postoperative 6 hours, 1 day and, 2 days (P < 0.05). But, no significant differences of their pain scores were observed between the groups. CONCLUSIONS: We conclude that the intraoperative supine position with both knees in flexion can reduced the incidence of postoperative low back pain in a fashion like using a lumbosacral wedge to support the lumbosacral area.