A prospective, randomized, double-blind trial was conducted to compare the analgesic actions and side effects of sufentanil continuously infused (5 micrograms/h) into the lumbar epidural space (L2-3) with those of an infusion of lumbar epidural morphine (0.5 mg/h). Forty patients admitted to an intensive care unit after elective major abdominal surgery participated over a varying period of 24-40 h. Post-operative pain was treated with an epidural bolus of either sufentanil (50 micrograms) or morphine (5 mg), followed by a continuous infusion of the same opiate. The quality of pain relief was similar in each group. The sufentanil group had a more rapid onset of analgesia. The incidence of nausea and vomiting, pruritus, and drowsiness was similar in the two groups. In spontaneously breathing patients there were no respiratory complications requiring treatment. Forced vital capacities were statistically significantly better during the first 1-4 h with sufentanil.