[Purpose] We prospectively investigated the effects of arthroplasty surgery on body weight and the effects of obesity on functional outcomes and complications in patients with total knee arthroplasty (TKA). [Subjects] One hundred eighty-six TKA patients were divided into two groups [non-obese body mass index (BMI) < 30 kg/m(2) (n=70), and obese = BMI >= 30 kg/m(2) (n=116)] according to preoperative BMI values. [Methods] Patients were evaluated preoperatively and at 12 months postoperatively with regard to BMI, knee function score, pain, range of knee flexion, and complications. There were no significant postoperative differences between the groups in pain, knee function score, range of knee flexion, and complications. [Results] A statistically significant postoperative improvement was found in all parameters (p<0.05) except for BMI. In the non-obese group, BMI was unchanged whereas in the obese group there was an increase in BMI after surgery. (Conclusion] The results of this Study indicate that patients do not lose weight after arthroplasty surgery. However, arthroplasty surgery offers significant improvements in knee flexion degree, knee function scores, physical function and relief of pain for patients regardless of weight. Increased mobility and decreased pain will not automatically lead to weight reduction. Therefore obesity must be addressed as an independent disease.