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The use of rotary instrumentation has increased dramatically in endodontics. Air-driven and electric handpieces are both currently available for use with the nickel-titanium files. To date no studies have compared the frequency of instrument separation and/or distortion while using the two types of handpieces with rotary files. The purpose of this study was to determine whether there is a difference in the frequency of file separation/distortion when an electric handpiece is compared with an air-driven handpiece. Extracted human molar teeth were instrumented with nickel-titanium Profile ISO .04 taper rotary files (Dentsply Tulsa Dental Products, Tulsa, OK) under controlled conditions and the results examined for statistical significance. The teeth chosen for the study fit the following criteria: minimal caries, curvature of any root no greater than 60 degrees and at least one root with a minimum of 30-degree curvature. The air-driven handpiece was used at 167.67 rpm and the electric handpiece at 150 rpm per manufacturer's recommendations. Files were examined both before and after use to evaluate whether breakage or deformation had occurred. There was no significant difference in file breakage or distortion when the handpieces were compared. There were, however, a significantly greater number of file separations/distortions noted in the #8 Profiles (ISO size 20) than with the other files. It may be prudent to view this file as a disposable instrument.