BACKGROUND:: The effects of fentanyl with ketamine for postoperative pain are unknown. We investigated the adjuvant effects of ketamine for fentanyl patient-controlled analgesia. METHODS:: Cervical and lumbar spine surgery patients were divided into three groups: ketamine 1 mg/kg followed by 42 and 83 μg • kg • h in ketamine-1 and ketamine-2 group, respectively, and a control group. Postoperative patient-controlled analgesia fentanyl was administered with a background infusion. RESULTS:: Pain scores and analgesia requirement in the ketamine-2 group were significantly lower than those of the control group after cervical surgery. Ketamine partially improved the analgesic effects of fentanyl after lumbar surgery. CONCLUSION:: Small-dose ketamine improved the analgesic effects of fentanyl after cervical surgery.