Aim: The purpose of this prospective study was to evaluate the prevalence of postoperative pain with respect to tooth vitality and preoperative pain in patients undergoing conventional root canal treatment. Methods: A total of 338 teeth from 317 patients, who were to receive conventional root canal treatment from four endodontists during a period of 18 months, were considered for this study. Based on the presence or absence of preoperative pain during the initiation of the treatment, the teeth were grouped as symptomatic (S) or asymptomatic (AS). Based on the presence or absence of haemorrhage during access cavity preparation, the teeth were grouped as vital (V) or non-vital (NV), respectively. The root canals were prepared using nickel-titanium ProTaper® rotary files and obturated using lateral compaction technique. The patients were recalled 1 week after the obturation for follow-up. At each visit, they were asked to record the postoperative pain as: none, slight, moderate or severe, since the previous visit. The data were statistically analysed using chi-square test. The level of significance was set at P 0.05. Results: 46 teeth were eliminated from the analysis of the results. Of the 292 remaining teeth, 66 fell into the non-vital asymptomatic (NV/AS) group, 60 in the non-vital symptomatic (NV/S) group, 39 in the vital asymptomatic (V/AS) group and 127 in the vital symptomatic (V/S) group. The prevalence of postoperative pain was significantly lower in the V/AS group compared to the NV/S (P = 0.002) and the V/S group (P = 0.029). The prevalence of preoperative pain was significantly higher in the NV group as compared to the V group (P = 0.049). The prevalence of postoperative pain was also significantly higher in the S group as compared to the AS group (P = 0.01). Conclusion: Pulp status and preoperative pain can be considered as factors that influence the occurrence of postoperative pain. Patients presenting with preoperative pain are at a higher risk of postoperative pain, particularly when the tooth is non-vital. Keywords: flare-up, postoperative pain, preoperative pain, root canal treatment, tooth vitality