Introduction: The aim of this study was to compare the incidence of dentinal defects after retreatment procedures with different nickel-titanium rotary retreatment files. Methods: One hundred-eighty mandibular premolars were randomly assigned to 6 groups (n = 30 teeth per group). One group was left unprepared, and the remaining 5 groups were prepared with K-files (Dentsply Maillefer, Ballaigues, Switzerland) and filled with gutta-percha and AH plus sealer (Dentsply Maillefer, Ballaigues, Switzerland). Of these 5 groups, 1 group was left filled and received no further treatments; in the other groups, removal of the filling material was performed with Mtwo R (VDW, Munich, Germany), D-RaCe (FKG Dentaire, La Chaux-de-Fonds, Switzerland), R-Endo instruments (Micro-Mega, Besancon, France), or Hedstrom files (Dentsply Maillefer). Roots were then sectioned 3, 6, and 9 mm from the apex and observed under a microscope. The presence of dentinal defects was noted. Chi-square tests were performed to compare the incidence of dentinal defects between the groups. A Pearson correlation test was performed to check the correlation between defects and root level or remaining dentin thickness. Results: No defects were observed in the unprepared group. Dentinal defects were detected in all retreatment groups. Retreatment, groups showed significantly more defects than the filled but no retreatment group (P < .05). There was no significant difference in the occurrence of defects between retreatment groups (P > .05). There was no correlation between the appearance of defects and level of the root or remaining dentin thickness. Conclusions: Under the experimental conditions, all retreatment techniques used in this' study created defects in the root dentin.