OBJECTIVE Patient-controlled analgesia (PCA) with morphine is commonly used to provide analgesia following major surgery, but is not sufficient as a monotherapy strategy. This study aimed to compare the adjunctive analgesic effect of ketamine versus tramadol on postoperative analgesia provided via PCA-morphine in patients undergoing major upper abdominal surgeries. METHODS Forty-two patients undergoing elective major upper abdominal surgery under general anesthesia were allocated to receive either ketamine (load dose of 0.5 mg kg-1 followed by a continuous infusion of 0.12 mg kg-1 h-1 up to 48 postoperative hours; ketamine group, n = 21) or tramadol (load dose of 1 mg kg-1 followed by a continuous infusion of 0.2 mg kg-1 h-1 up to 48 postoperative hours; tramadol group, n = 21) in addition to their standard postoperative analgesia with PCA-morphine. Postoperative data included morphine consumption, visual analog scale (VAS) scores, and side effects during the first 48 postoperative hours after PCA-morphine initiation. RESULTS There were no significant differences in patient demographic and intraoperative data between the two groups. Tramadol group had significantly less total morphine consumption during the first 48 postoperative hours (28.905 [16.504] vs 54.524 [20.846] mg [p < 0.001]) and presented significantly lower VAS scores at rest and mobilization (p < 0.05) than the ketamine group. No statistical difference was recorded between the two groups (p > 0.05) regarding postoperative cough, sedation, hallucinations, pruritus, urine retention, and postoperative nausea and vomiting. However, patients in the ketamine group reported dry mouth more frequently than patients in the tramadol group (p = 0.032). CONCLUSIONS Postoperative administration of tramadol was superior to ketamine due to significantly reduced opioid consumption and better pain scores in patients receiving PCA-morphine after major upper abdominal surgery.