The cam-roller system, commonly used in heavy duty truck applications, consists in a roller follower actuated by the engine camshaft to transmit the cam linear movement. At the application under study, the contact to the cam is through a roller made of 100Cr6 bearing steel. In a Dynamometer test, two out of six rollers presented an unacceptable level of wear, characterized by formation of pitting and spalling, typical of contact fatigue wear. Plastic deformation caused by high friction sliding was also detected by SEM analysis. A macrography of roller's top surface, polished and etched with Nital, shown discrete regions where sliding occurred, with non-tempered martensite spots surrounded by severely tempered martensite regions, evidencing intermittent sliding and rolling movements. The wear mechanism correlates with information from literature about "plasticity-dominated sliding wear" caused by high friction sliding between surfaces, leading to material removal by contact fatigue through two different coexistent mechanisms. Other possible causes of failure were discarded, hence it can be stated that the wear cause is inadequate lubrication of engine oil, such as low lubricity and particles in suspension, associated with unequal thermal expansion, studied by FEA, leading to roller/cam sliding and resulting in plastic deformation, microstructure changes on roller surface and particles detachment by low cycle contact fatigue failure at the end of the test.