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Abstract: The purpose of this study was to compare apical transportation and changes in canal curvature of three nickel-titanium (Ni-Ti) rotary instruments: Hero 642, RaCe, and ProTaper. Mesiobuccal canals of 60 maxillary first molars (with angles of curvature between 25 degrees and 35 degrees) were prepared with a torque-control, low-speed engine. Canals were prepared using the crown-down technique to the size of #30. Using a radiographic platform, reproducible preinstrumentation and postinstrumentation radiographs were taken. A computer analysis allowed magnification and superimposition of the images. The central axes of initial and final instruments were radiographically superimposed to measure transportation at 1 mm from WL. Data were analyzed using repeated-measures ANOVA. A statistically significant difference in apical transportation was found in ProTaper group. The results suggest that ProTaper file system be implemented in combination with other less tapered more flexible systems, like RaCe, in preparing curved canals. [Copyright &y& Elsevier]