Introduction: Obesity is generally considered a chronic metabolic disease which is characterised by excessive accretion of adipose tissue. This pathology of fat accretion may lead to creation of pathological units, including gonarthrosis. The aim of the study was to evaluate the impact of body weight on functional performance in early stages of rehabilitation after knee replacement. Material and methods: The research material comprised 35 patients. The Group included 26 women and 9 men who suffer from advanced degenerative changes of knee joint and qualified for the knee replacement. The patients were assigned to one out of three Groups depending on body mass index. To assess the functional status the following tools were used: the VAS scale, 100-point Lysholm-Gillquist scale, the TUG test, and the measurement of the range of joint mobility. Results: Statistically significant results were achieved by first and second Group in joints mobility measurement. All the Groups revealed statistically meaningful differences in pain measurement with the use of VAS scale and 100-point Lysholm-Gillquist scale. Only third Group proved statistically meaningful outcomes while TUG test was used in the assessment. Conclusion: High body mass index does not have a negative impact on the re-establishment of functional ability after knee replacement. In the assessment of selected parameters (ROM, TUG), people who had BMI >35,00 kg/ m2 got better results than people who had lower BMI.