Separation of instruments while performing root canals is something that has plagued all practitioners. The purpose of this study was to evaluate the effect of rotational speed on nickel-titanium file distortion and separation. Forty extracted human molars were instrumented with Profile (Dentsply Tulsa Dental Products, Tulsa, OK) ISO .04 rotary nickel-titanium instruments at 166.67 or 333.33 rpm. The instruments were evaluated after instrumentation for separation and/or distortion. The results of this study were statistically significant and indicate that file distortion and/or separation is four times as likely to occur at 333.33 rpm vs. 166.67 rpm.