Background:Pain relief after surgery is an essential component of postoperative care.Objectives:The purpose of this study was to compare the efficacy of intravenous acetaminophen and intravenous meperidine in pain relief after outpatient urological surgery.Patients and Methods:In a prospective, randomized, double-blind clinical trial, 100 outpatients of urological surgery were studied in two groups of acetaminophen (A) and meperidine (M). Patients in group A received 1g of acetaminophen in 100 mL saline within 15 minutes and patients in group M received a single intravenous injection of meperidine 0.5 mg/kg, 15 minutes prior to the end of operation. Postoperative pain was recorded using visual analog scale (VAS). Vital signs, nausea, vomiting, dizziness and respiratory depressions were compared between the two groups.Results:Pain severity in patients treated with intravenous acetaminophen six hours after the operation within one-hour interval was significantly lower than meperidine group (P < 0.0001). Ninety patients in the meperidine group and five patients in the acetaminophen group required additional doses of analgesics. Nausea was significantly lower in acetaminophen group than meperidine group.Conclusions:Intravenous acetaminophen reduced pain following outpatient urological surgery more significantly than meperidine.