BACKGROUND: The use of a low-dose naloxone infusion concomitant with intravenous morphine PCA (patient-controlled analgesia) attenuates opioid-related side effects without reducing analgesic effects. The authors compared the incidence of morphine-related side effects and the quality of analgesia in adding low-dose naloxone, normal saline or droperidol to an IV fentanyl PCA regimen. METHODS: One hundred eight patients undergoing ocular plastic surgery were enrolled in the study. General anesthesia was induced and maintained with propofol TCI (target controlled infusion), vecuronium or pancuronium and nitrous oxide. After intubated, they received intravenous fentanyl as PCA. They were randomized to receive normal saline, droperidol or low-dose naloxone concomitant with IV fentanyl PCA. Verbal rating scores for pain, the degree of patients' satisfaction (1-4), nausea, vomiting, requests for antiemetics, urinary retention, pruritus and respiratory depression were recorded after 24 hours. RESULTS: There was no difference in the VRS (verbal rating score) for pain, degree of the satisfaction and the incidence of nausea, vomiting and requests of antiemetics among the three groups. There was no incidence of pruritus or respiratory depression. One subject developed urinary retention in the control group, and three cases in the droperidol group, but none was developed in the low-dose naloxone group. CONCLUSIONS: There was no difference in the prevention of postoperative nausea, or vomiting among the normal saline, droperidol, and naloxone groups with an IV fentanyl PCA. Low-dose naloxone, however, had a reducing effect on urinary retention; it may become an alternative choice according to the anesthesiologist's preference.