Importance: Intravenous acetaminophen is a commonly prescribed analgesic for the prevention and treatment of postsurgical pain. Its efficacy in the context of endoscopic sinus surgery (ESS) has yielded mixed results. Objective: To compare the efficacy of perioperative intravenous acetaminophen (IVAPAP) with that of placebo in improving early postoperative pain after endoscopic sinus surgery (ESS). Design, Setting, and Participants: A prospective, randomized clinical trial including 62 patients undergoing ESS for chronic rhinosinusitis in a single tertiary referral hospital. Interventions: Participants were randomized to receive 1 g of IVAPAP or 100mL of placebo consisting of saline infusions immediately before the start of surgery and 4 hours after the initial dose. Main Outcomes and Measures: The primary outcomewas postoperative pain measured by visual analog scale (VAS) scores up to 24 hours after surgery by blinded observers. Secondary endpoints included postoperative opioid (intravenous and oral) use and adverse events in the 24-hour postoperative period. Results: Of the 62 enrolled adult participants, 60 were randomized (31 to IVAPAP intervention and 29 to placebo). The mean (SD) age of participants was 53.7 (14.7) years and 35 (58%) of the participants were men and 25 (42%) were women. Within the first hour, mean pain scores were reduced in the IVAPAP group compared with the control group, reaching a maximum difference of 7.7mmon a VAS scale favoring the treatment group with a true difference possibly as high as 22 mm, and the data are compatible with a clinically meaningful difference. At 12- and 24-hours, average pain scores were less in the placebo group and the data are compatible with a clinically meaningful difference of 5.8 (-5.2 to 16.8) and 8.2 (-1.9 to 18.4), respectively, favoring the placebo group. However, at all time points the CIs included the null value and were wide, thus preventing definitive conclusions. Inspection of the secondary outcomes favored IVAPAP, but the wide range of the CIs and inclusion of the null value prevent definitive conclusions. Conclusions and Relevance: The results of this study are inconclusive. The data suggest that perioperative intravenous acetaminophen may reduce immediate postoperative pain and opioid requirements compared with placebo and these differences could be clinically meaningful. Unfortunately, the imprecision of the estimates prevents definitive conclusion. Use of IVAPAP does not seem to increase adverse events.