Thirty-eight ASA I–III patients undergoing lower abdominal operations were randomly allocated to receive either morphine (group M, patient-controlled analgesia bolus=1 mg of morphine) or tramadol (group T, patient-controlled analgesia bolus=10 mg of tramadol) for post-operative patient-controlled analgesia (PCA) after receiving morphine intraoperatively. There were no between-group differences in the pain, sedation or vomit scores. The nausea scores were significantly higher in group T in the initial 20 h and between 32 and 36 h (P<0.01, 0–4 and 8–12 h; P<0.05, 4–8, 12–16, 16–20 and 32–36 h). The incidence of dizziness was also significantly higher in group T (68.4% vs. 31.6%, group T vs. group M, P<0.05). There was no difference in the overall satisfaction. We conclude that the use of tramadol, compared with morphine, for post-operative PCA after intraoperative loading with morphine is associated with more nausea and dizziness, but with similar sedation, quality of analgesia and patient satisfaction.