Objective:This randomized clinical trial evaluated the effect of passive ultrasonic irrigation (PUI) protocol employed during endodontic treatment on the incidence of postoperative pain. Methods: Sixty adult patients with non-vital asymptomatic premolar and molars were included. The treatments were performed using ProTaper Universal instruments with working length set 1.0 mm short of the apical foramen. The patients were randomly divided to receive one of the following two final cleansing protocols (n = 30): PUI - the canals were rinsed with 2.5% NaOCl, 17% EDTA, and 1.0% NaOCl sequentially, with 3 cycles of 20 seconds in each irrigation solution, under ultrasonic agitation; Control (CON) - similar final irrigation protocol was employed without any agitation. To evaluate postoperative pain, a numerical rating scale was applied every 24 hours until the fifth day. Data were statically analyzed with Pearson's Chi-square, Friedman's and ANOVA tests with 5% of significance. Results: There were no significant differences in any of the periods (P=0.46). The great majority of patients in both groups showed no pain symptoms. No patient reported severe pain (≥ 7), with moderate pain occurring in only 2 patients in the CON group (7.4%) and 4 patients in the PUI group (16.7%), exclusively in the first 24 hours. Conclusions: The use of passive ultrasonic irrigation protocol did not influence the occurrence of postoperative pain in patients submitted to endodontic treatment.