Long-term storage of polymer materials in adverse conditions initiates ageing process. That process covers all physical and chemical conversions taking place in the structure and effecting in change of parameters of the material. This paper presents the results of the authors' own research to determine changes in the physical properties of the polymer (UHMWPE) under the influence of: ionising radiation, long-term compressive loads, temperature, the effects of the use environment (human body fluids) and polymer storage time. The in vitro degradation process of UHMWPE polyethylene under the influence of the above-mentioned factors was evaluated. Assumptions were made to create test conditions for the samples similar to those found in the human body. There have also been conducted Shore’s D tests on hardness as well as DSC analysis.