The objective of this study was to compare the total knee arthroplasty (TKA) functional outcomes and quality of life of obese and non-obese patients. Prospective comparative study, including all patients underwent TKA in a single centre. Patients were divided into three groups: Group 1 (Gr.1) BMI <30 kg/m2, Group 2 (Gr.2) BMI ≥ 30 kg/m2 and <35 kg/m2 and Group 3 (Gr.3) BMI ≥35 kg/m2. The Knee Society score (KSS) and SF-36 scores were obtained preoperatively and at 5 years of follow-up. A total of 689 patients were included (72.2 ± 7 years, 76.3 % women). Overall, pre- and post-operative values of SF-36 were lower for the obese group. However the improvement obtained in the three groups was similar in all the sub-scales of the SF-36 score. KSS values were higher in the non-obese group in both periods. However the improvement obtained in this score in the three groups was similar (Gr.1; 70.21 ± 34.31, Gr.2; 66.53 ± 34.93, Gr.3; 60.94 ± 38.47, n.s.). Although non-obese patients obtained better functional and reported quality of life scores than obese patients, there were no differences in the gain of quality of life and knee functionality between both groups at 5-years of follow-up. This is one of the largest series in a single centre published in literature and confirms the results obtained by other authors. Taking into account the different outcomes obtained, surgery should not be denied to patients that are obese, given that they obtained similar benefit than non-obese patients.