Introduction: The purpose of this study was to evaluate the effect of various degrees of cyclic fatigue on torsional failure and torsional preloading on the cyclic fatigue life of heat-treated K3XF nickel-titanium (NiTi) instruments (SybronEndo, Orange, CA). Methods: The mean number of cycles until failure (N-f) of K3XF and K3 NiTi instruments was examined in a 3-point bending apparatus with a 7-mm radius and 45 curve. Torque and distortion angles at failure of new instruments and instruments stressed to 25%, 50%, and 75% of the N-f were measured according to ISO 3630-1. Other new files were preloaded at 25%, 50%, and 75% of the mean distortion angles before the fatigue test. After torsional preloading, the N-f was examined. The fracture surface of each fragment was examined with a scanning electron microscope. Results: The fatigue resistance of K3XF instruments was 2 times higher than that of K3 instruments (P < .05). The torque and angle of rotation at fracture of K3XF instruments were similar to those of K3 instruments. The 25%, 50%, and 75% torsional preloading significantly lowered the N-f of both K3 and K3XF instruments (P < .05). In the fatigue prestressed groups, K3 instruments with 75% preloading had significantly lower torque and distortion angles than unused K3 instruments (P < .05). The fractographic patterns corresponded to the pattern defined by the last stage test. Conclusions: A low amount of torsional preloading reduced the fatigue resistance of K3 and K3XF instruments. A high amount of precycling fatigue ignificantly reduced the torsional resistance of K3 instruments. The torsional resistance of K3XF instruments was less affected by previous load cycling even after extensive precycling.