It is shown in this study that polymer morphology, sterilization with or without accelerated aging, and temperature affect the fatigue crack propagation behavior of ultra high molecular weight polyethylene (UHMWPE). Changes in UHMWPE orthopedic implants due to sterilization and shelf aging are of great clinical interest and importance. In recent years, accelerated aging has been used to simulate shelf aging and post-sterilization oxidation in UHMWPE orthopedic components. In order to understand the role of sterilization and post-sterilization aging, two commercially available polymers (GUR4150HP and Hylamer(R)-M) with distinct morphologies were subjected to one of four sterilization techniques (gamma radiation in air, gamma radiation in vacuum, ethylene oxide gas, and gas plasma). Non-sterile samples were used as controls for all studies. Half the groups were tested in the unaged condition. Time remaining samples were aged under accelerated conditions via elevated temperature (70 degrees C) and oxygen pressure (5 atm) for 14 days to simulate five years of shelf aging. Subsequent to sterilization and aging treatments, specimens were subjected to cyclic compression loading followed by cyclic tensile loading in order to characterize the fatigue resistance. Scanning electron microscopy was used to provide an understanding of fatigue fracture mechanisms. Fourier transform infrared spectroscopy and density gradient column methods were used to determine the structural changes brought about by sterilization and aging. It is found in this work that accelerated aging causes a decrease in fatigue resistance regardless of sterilization method. Fatigue degradation is most severe for gamma radiation in air coupled with accelerated aging conditions. The results of this study have important implications for the design of fatigue resistant polymers used in medical applications.