Ultra-high molecular weight polyethylene (UHMWPE) is extensively used in total joint replacements. Wear, fatigue, and fracture have limited the longevity of UHMWPE components. For this reason, significant effort has been directed towards understanding the failure and wear mechanisms of UHMWPE, both at a micro-scale and a macro-scale, within the context of joint replacements. We have previously developed, calibrated, and validated a constitutive model for predicting the loading response of conventional and highly crosslinked UHMWPE under multiaxial loading conditions (Biomaterials 24 (2003) 1365). However, to simulate in vivo changes to orthopedic components, accurate simulation of unloading behavior is of equal importance to the loading phase of the duty cycle. Consequently, in this study we have focused on understanding and predicting the mechanical response of UHMWPE during uniaxial unloading. Specifically, we have augmented our previously developed constitutive model to also allow for accurate predictions of the unloading behavior of conventional and highly crosslinked UHMWPE during cyclic loading. It is shown that our augmented hybrid model accurately captures the experimentally observed characteristics, including uniaxial cyclic loading, large strain tension, rate-effects, and multiaxial deformation histories. The augmented hybrid constitutive model will be used as a critical building block in future studies of fatigue, failure, and wear of UHMWPE.