Abstract Objective To evaluate the effect of final irrigation of root canals with NaOCl solution at different temperatures on postoperative pain level and antimicrobial activity.

Methodology 45 patients were randomly divided into three groups using a web program according to the irrigation selected:

NaOCl 2ºC, NaOCl 25ºC and NaOCl 45ºC. First root canal samples were collected before treatment (S1). After chemo-mechanical preparation, final irrigation was performed with the selected irrigant (NaOCl 2ºC, NaOCl 25ºC and NaOCl 45ºC) and second samples were collected (S2). Samples were subjected to quantitative real-time polymerase chain reaction to evaluate the levels of total bacteria. The root canal treatments were completed and the participants were given instructions to record postoperative pain levels at 24, 48 and 72 hours, 5 days and 1 week after treatment using a visual analog scale (VAS). Results The reduction in the number of total bacterial cell equivalents from S1 to S2 was statistically significant in all groups (p<0.001). The NaOCl 2˚C group reported significantly less postoperative pain than the NaOCl 45˚C group (p<0.05). Postoperative analgesic intake was significantly higher in the NaOCl 45˚C group than in the NaOCl 2˚C group (p<0.05). Conclusion We conclude that final irrigation with NaOCl at different temperatures results in similar antibacterial effectiveness. Final irrigation with cold NaOCl (2˚C) is better than NaOCl 45˚C when comparing postoperative pain levels.