This paper presents the results of a joint research project by Federal-Mogul Corporation and Climax Molybdenum Company of Michigan to determine the effect that the presence of bainite in the outer carburized case has on rolling contact fatigue life.Rolling contact fatigue specimens (0.375-in. diameter bars) of AISI 4118 steel that had been carburized and then quenched at different rates to produce varying amounts of bainite in the outer case were used for this study. After fatigue testing, these specimens were sectioned and metallographically examined to determine the volume fraction of bainite present in the outer case for each test track. The hardness for each test track was also measured. This data along with the fatigue data was then analyzed statistically using the Weibull method to determine the effects of bainite in the carburized case on the rolling contact fatigue life.