The analgesic efficacy and safety of tramadol and morphine were compared in a double-blind, randomized study of 150 female patients after gynecologic surgery. As required, patients could receive up to three intravenous doses of either 50 mg of tramadol or 5 mg of morphine within a period of 6 h. Pain intensity (verbal response score) was recorded before injection and at 0.5, 1, 2, 3, 5, and 6 h after the initial dose; at these times, pain relief was also assessed. Oxygen saturation was monitored continuously by pulse oximetry for at least 30 min after each injection. In 13.3% of the morphine group (but in none of the tramadol group) transcutaneous pulse oxygen saturation decreased to less than 86%; in 50% of these patients the decrease occurred after only the first 5 mg of morphine. Both drugs produced acceptable analgesia, and there were no clinically significant adverse events. In demonstrating the absence of clinically relevant respiratory depression with tramadol, we underline its safety for postoperative pain relief.