Four types of solidrotational nickel-titanium endodontic instruments: LightSpeed size 35 (LightSpeed Technology, USA), RaCe size 20 0.02 (FKG Dentaire, Switzerland), FlexMaster size 20, 0.02 taper (VDW, Germany), PathFile 019 (DentsplyMaillefer, Switzerland) and a prototype non-solid rotational steel instrument were investigated. Fatigue testing was carried out in specially prepared, transparent glass tubes. Tubes were bent in such a way that the instrumentdeﬂection angle was 90°, and radius of curvature was 5 mm. Endodontic instruments were placed in the tubes so that their diameter at the point of maximum curvature was 0.3 mm. Rotation speed was 300 rpm. Time to fracture was recorded and converted to number of cycles for each instrument.