BACKGROUND: Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, is known to inhibit "wind-up" and hence central hyperexcitability of dorsal horn neurons. However the results of clinical studies for its preemptive analgesic effect are controversial. The object of this study is to evaluate the effects of preincisional and postincisional low-dose ketamine on postoperative pain. METHODS: In a randomized, double-blind study, postoperative pain was assessed in 60 patients undergoing spinal fusion with general anesthesia who were allocated to three groups. Twenty patients were received 0.15 mg/kg of ketamine and the same volume of saline 5 min before and 15 min after surgical incision, respectively (group I). Patients in group II received 0.15 mg/kg of ketamine and the same volume of saline 15 min after and 5 min before surgical incision, respectively (n = 20), and in control group, patients received saline 5 min before and 15 min after surgical incision (n = 20). IV patient-controlled analgesia (PCA) with a morphine-ketorolac mixture was started in all patients at skin closure. Visual numerical scale (VNS) pain score, total analgesic consumption, and side effects were recorded at 1, 3, 6, 12, 24 and 48 h postoperatively. RESULTS: No significant intergroup differences were seen in the VNS pain scores, total analgesic consumption and incidence of side effects at 1, 3, 6, 12, 24 and 48 h postoperatively. CONCLUSIONS: This result indicates that postoperative pain cannot be decreased when ketamine in low doses is added to general anesthesia before and after surgical stimulation.