BACKGROUND: The lockout interval is a safe guard to prevent patients from taking additional dose before the full effect of the preceding dose. Therefore, it should correlate with the time-to-peak effect of the opioid selected. The time-to-peak effect of meperidine is known to be different from that of morphine and fentanyl. But there have been few reports about the influence of varying lockout interval on IV-PCA using meperidine. So we studied the influence of varying lockout interval with constant hourly maximum dose on IV-PCA using meperidine. METHODS: This study included sixty patients undergoing low abdominal surgery under general anesthesia. After administration of initial dose of meperidine (0.5 mg/kg) they were randomly assigned to three groups according to the lockout interval; Group 1 (6-min lockout interval, 0.2 mg/kg bolus dose), Group 2 (9-min lockout interval, 0.3 mg/kg bolus dose), Group 3 (12-min lockout interval, 0.4 mg/kg bolus dose). We examined NRS pain score, sedation score, satisfaction score, PCA measurements and the incidence of side effects during 24 hours. RESULTS: There were no significant differences in NRS pain score, sedation score, satisfaction score, the amount of meperidine consumed, injections/attempts ratio and the incidence of side effects among three groups. The numbers of injections and attempts were significantly higher in Group 1 than in Group 2 and Group 3 (P<0.05). CONCLUSIONS: The lockout intervals chosen for this study (6-min, 9-min, 12-min) do not influence pain, side effects, satisfaction and meperidine consumption in IV-PCA using meperidine when hourly maximum dose is constantly 2 mg/kg.