Gears are frequently exposed to conditions involving random or repeated high strain rate loading in addition to high cycle bending fatigue stresses. Although gears usually fail ultimately by high cycle fatigue, random impact loads superimposed on the predictable bending fatigue stress may lead to premature fatigue failure. The present study examined the effect of repeated prior impact loading on the high cycle fatigue properties of carburized 4118, 4820 and EX-1 steels. It was found that impact prestressing does affect the endurance limit and that this effect can either be detrimental or beneficial. Case microstructure, particularly retained austenite content, and nickel content both affected sensitivity to impact prestressing. The greatest resistance to impact fatigue damage was found in EX-1 steel which had been heat treated to contain approximately 20% austenite. The results of this study have been interpreted in terms of the Troiano theory of ductility in high carbon alloy steels.