The effects of microscopic discontinuities on reducing the fatigue strength of steels are well known. This paper will attempt to quantify the reduction of fatigue strength due to measured quantities of microvoids, oxide inclusions, and sulfide inclusions of the fatigue strength of wheel steels. Murikami and Endo have derived a model that relates the effects of void and inclusion area and hardness on the fatigue endurance limit. In the present work is presented a unique analysis where the effect of microcleanliness is investigated. The Murikami model was used for simplicity and because the goal in this publications is to demonstrate that inclusions play an important role on wheel performance. This work summarizes the microcleanliness analysis of 113 wheels; most of such wheels were removed from revenue service due to failure. The nature of the failure indicates that the wheels failed due to a fatigue related problem. For these reason, it is of paramount interest to determine the effects of defects (i.e. voids or inclusions) on wheel performance. For this reason a simple model (Murikami) was employed to quantify the effect of defects that were measured using the microanalysis.Copyright © 2007 by Transportation Technology Center, Inc.