The use of ferrography to monitor the health of helicopter gearboxes undergoing fatigue substantiation trials is described. The results illustrate the sensitivity of ferrography in detecting the onset of gear fatigue and indicate that the fatigue was initiated by micropitting. This suggests that the gears were not properly lubricated under the fatigue loads.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference in San Francisco, California, August 18–21, 1980.