Aim: To compare the effects of two different concentrations of NaOCl solution on postoperative pain following single-visit root canal treatment in mandibular molars with irreversible pulpitis. Methodology: A total of 122 patients who had mandibular molars with irreversible pulpitis were treated. The patients were randomly divided into two groups according to the concentration of NaOCl used during root canal instrumentation - either 2.5% or 5.25%. RaCe rotary instruments were used for root canal preparation, and all root canals were filled in one visit. Postoperative pain was evaluated using the visual analogue scale. Data were analysed by independent t-test, chi-square and Mann-Whitney tests. Results: Twelve patients were excluded for various reasons. Pain reported by 110 patients who were eligible to be included in the study was analysed. No significant differences were found in the age and gender of the patients between the two groups (P = 0.50, P = 0.51, respectively). The patients who had 5.25% NaOCl reported significantly lower postoperative pain compared to those who had 2.5% NaOCl during the first 72 h following treatment (P = 0.021); however, there was no significant difference in pain felt by the patients during the rest of the study period, that is 4-7 days following treatment (P = 0.185) when the four-level pain categorization method was used. When the two-level pain categorization method was used, the results revealed that patients who had 5.25% NaOCl reported significantly less pain for the first 3 days after treatment (P = 0.026). The number of analgesics taken by patients who had 2.5% NaOCl was significantly higher than that taken by patients who had 5.25% NaOCl (P = 0.001). Conclusion: 5.25% NaOCl was associated with significantly lower postoperative pain compared to 2.5% NaOCl during the first 72 h following one-visit root canal treatment of mandibular molars with irreversible pulpitis.