Discarded ProFile and ProTaper nickel-titanium rotary instruments, with unknown history of clinical use, were obtained from graduate endodontic clinics at Ohio State University and University of Texas Health Science Center at Houston Dental Branch. These discarded instruments and as-received instruments of both types were examined with a scanning electron microscope to investigate effects of clinical use and causes of failure. For used ProTaper instruments, dentinal debris was wedged mostly in narrow, radial, land-type regions and less on convex flute surfaces. For used ProFile instruments, dentinal debris was wedged mostly in the metal rollover and on concave flute surfaces. Used instruments of both types exhibited widened machining grooves, and elongated and stretched roll-over. Dentin chips were wedged in surface micro-cracks that appeared to propagate from original machining flaws and widen during in vivo root canal preparation. From our observational study, wedged dentinal deposits seem to play a pivotal role for clinical failure of these instruments.