AimTo examine the fatigue behaviour of heat-treated NiTi instruments when immersed in aqueous media and to determine the effect of cyclic fatigue on the hardness and elastic modulus of NiTi instruments using a nanoindentation technique. MethodologyK3XF and K3 NiTi instruments, both in sizes 25, 0.04 taper and 40, 0.04 taper, were subjected to rotational bending at a curvature of 42 degrees either in air or under deionized water, and the number of revolutions to fracture (N-f) was recorded. The fracture surface of all fragments was examined with a scanning electron microscope. The hardness and elastic modulus of the fracture surface of instruments sized 25, 0.04 taper were then measured using a nanoindentation test. ResultsThe K3XF instruments had a fatigue resistance superior to K3 instruments under dry and aqueous environments (P<0.05). The fatigue life of K3 instruments was similar under both conditions, whereas the N-f of K3XF was greater under water than in air, especially at the size 40, 0.04 taper (P<0.05). The values for the fraction of the area occupied by the dimple region were significantly smaller in K3XF instruments than in K3 instruments, especially under water (P<0.05). There was no difference in hardness on K3XF instruments between new files and instruments subjected to the fatigue process. The hardness of instruments subjected to the fatigue process was significantly lower in K3XF than in K3 instruments (P<0.05). ConclusionThe fatigue life of K3XF instruments under water is longer than it is for K3XF instruments in air. There was no work-hardening effect on K3XF instruments subjected to the fatigue process.