BACKGROUND: In this study, we assessed the effectiveness of ketamine as an alternative to non-steroidal anti-inflammatory drugs (NSAID), to manage acute postoperative pain after spinal fusion when given intravenously via a patient-controlled analgesia (PCA) pump in which the dose was proportional to that of fentanyl. METHODS: Forty patients undergoing 1-2 level spinal fusion were enrolled in this study. Patients were intraoperatively randomized into two groups to receive intravenous PCA consisting either of fentanyl 0.4 microg/ml/kg (control group) or fentanyl 0.4 microg/ml/kg with ketamine 30 microg/ml/kg (ketamine group) after intravenous injection of a loading dose. The loading dose in the control group was fentanyl 1 microg/kg with normal saline equal to ketamine volume and in the ketamine group it was fentanyl 1 microg/kg with ketamine 0.2 mg/kg. The verbal numerical rating scale (NRS), fentanyl and ketamine infusion rate, and side effects were evaluated at 1, 24, and 48 hours after surgery. RESULTS: There were no significant differences in patient demographics, duration of surgery and anesthesia or intra-operative opioids administration. We did not find any significant differences in the mean infusion rate of intraoperative remifentanil or postoperative fentanyl or in the side effects between the groups, but we did find a significant difference in the NRS between the groups. CONCLUSIONS: Based on our results, we conclude that a small dose of ketamine (0.5-2.5 microg/kg/min) proportional to fentanyl is not only safe, but also lowers postoperative pain intensity in patients undergoing spinal fusion, although the opioid-sparing effects of ketamine were not demonstrated.