ProFile nickel-titanium Gates Glidden drills were tested in bending fatigue to simulate clinical conditions. Ten samples each in sizes #1 through #6 were placed in a device that deflected the drill head 4 mm from the axis. The drill head was placed inside a ball bearing fixture, which allowed it to run free at 4000 rpm, and the total number of revolutions was recorded until failure. Fracture surfaces were examined with a scanning electron microscope to determine the initiation site and nature of the failure process. Mean +/- SD for the number of revolutions to failure for the drill sizes were: #1: 1826.3 +/- 542.5; #2: 5395.7 +/- 2581.5; #3: 694.4 +/- 516.8; #4: 261.0 +/- 138.0; #5: 49.6 +/- 14.9; #6: 195.9 +/- 78.5. All drills failed in a ductile mode, and fracture initiation sites appeared to be coincident with machining grooves or other flaws, suggesting the need for improved manufacturing procedures.