We designed this prospective, randomized, double-blind study to compare the analgesic effectiveness and side effects of IV patient-controlled morphine, pethidine, and tramadol for postoperative pain management. One-hundred-twenty-six ASA physical status I or II patients undergoing abdominal hysterectomy were randomly allocated to receive IV-patient controlled morphine (M), pethidine (P), or tramadol (T) for postoperative analgesia. The cumulative analgesic consumption over 24 h was 25.7 +/- 9.5 mg for morphine, 266 +/- 90 mg for pethidine, and 320 +/- 10 mg for tramadol. The average supplementary fentanyl dose used was significantly higher in group T than in groups M and P (P < 0.05). In conclusion, morphine, pethidine, and tramadol resulted in equivalent pain scores and side effects, but tramadol group T required more rescue analgesic doses of fentanyl.