Background Although acetaminophen has been used widely and is well tolerated in children, its efficacy and safety have not been clarified when combined with an opioid in intravenous parent-/nurse-controlled postoperative analgesia. Methods Sixty-three children (aged 6-24 months) who had undergone elective ureteroneocystostomies were enrolled in this prospective, randomized, double-blinded study. After the surgery, an analgesic pump was programmed to deliver fentanyl at a basal infusion rate of 0.25 microg.kg(-1).h(-1) and 0.25 microg/kg bolus after a loading dose of 0.5 microg/kg(-1). In the fentanyl-acetaminophen group, acetaminophen was coadministered as a solution mixture at a basal infusion rate of 1.5 mg.kg(-1).h(-1) and 1.5 mg/kg bolus after a loading dose of 15 mg/kg, whereas saline was administered to the fentanyl group. Results Postoperative pain scores were similar between the two groups. The total dose (micrograms per kilogram per day, mean+/-SD) of fentanyl at postoperative days 1 (8.3+/-3.7 vs. 18.1+/-4.6, P=0.021) and 2 (7.0+/-2.4 vs. 16.6, P=0.042) was significantly less in the fentanyl-acetaminophen group compared with that in the fentanyl group. The incidences of vomiting (16.1 vs. 56.3%, P=0.011) and sedation (9.7 vs. 46.9%, P=0.019) were significantly lower in the fentanyl-acetaminophen group than those in the fentanyl group. Conclusions Acetaminophen has significant fentanyl-sparing effects and reduces side effects when combined with fentanyl in intravenous parent-/nurse-controlled analgesia for postoperative pediatric pain management.