The purpose of this study was to analyze the type and frequency of defects in nickel-titanium rotary endodontic files after routine clinical use, and to draw conclusions regarding the reasons for failure. All of the files (total: 378, Quantec Series 2000) discarded after normal use from a specialist endodontic practice over 6 months were analyzed. Almost 50% of the files showed some visible defect; 21% were fractured and 28% showed other defects without fracture. Fractured files could be divided into two groups according to the characteristics of the defects observed. Torsional fracture occurred in 55.7% of all fractured files, whereas flexural fatigue occurred in 44.3%. The results indicated that torsional failure, which may be caused by using too much apical force during instrumentation, occurred more frequently than flexural fatigue, which may result from use in curved canals.