Introduction: One of the most common complications of root canal treatment is postoperative pain. The aim of the present clinical trial was to compare the severity of postoperative pain after root canal preparation with RaCe rotary system and hand K-Flexofile. Methods and Materials: A total of 96 mandibular first and second molars were divided into two groups (n=48) based on root canal preparation technique. The teeth in both groups underwent one-session root canal treatment and the severity of postoperative pain was evaluated using visual analog scale (VAS) at 4-, 8-, 12-, 24- and 48-h and 1-week intervals. In addition, the type and dosage of analgesics were recorded. Data were analyzed with repeated-measures ANOVA. Statistical significance was set at 0.05. Results: The difference between the two groups during this period and at subsequent intervals were not significant (P>0.05). There were no significant differences between the two groups in type and the number of analgesics in pain-free subjects (P=0.12 and P=0.61, respectively). Conclusion: There were no statistically significant differences in pain severity between the two groups at any intervals.Keywords: Manual Instrumentation; Postoperative Pain; Root Canal Preparation; Rotary Instrumentatio
