OBJECTIVES:Total hip and knee replacement are very common procedures. Both of them improve function, pain and health related quality of life (HRQoL). The aim of this study was to compare baseline scores and improvements at one year in HRQoL according to the operated joint METHODS: This prospective observational study was carried out in 15 hospitals with consecutive patients. All included patients fulfilled 3 HRQoL questionnaires at two times, 1 month pre surgery and 12 months post surgery. There were 2 generic questionnaires; EQ-5D and SF-12 and one specific, the WOMAC. We compare the baseline scores and improvements at one year by mean of Student’s t test. RESULTS: The analysis was performed over 864 patients who fulfilled the included questionnaires. There were 355 hips and 509 knees. Mean age (mean ± s.d.) was not different between hip (63.5±19.9) and knee (64.3±23.8) patients. At baseline, patients who were waiting for hip replacement had only worse score in the function dimension of WOMAC (64.8±16.6 vs 61.1±17.6) (p = 0.002). According to the improvements at one year, there were statistically significant differences in all EQ-5D and WOMAC dimensions. The patients in the hip group experienced higher improvements in EQ-5D index (p =0.005), VAS (p = 0.005), WOMAC pain (p <0.001), function (p = 0.001) and stiffness (p <0.001). There were no statistically differences in the improvements measured by SF-12. CONCLUSIONS:In the light of these data we can conclude that at baseline patients have similar HRQoL scores, while at 1 year those who have undergone hip replacement experienced a higher improvement in the affected dimensions. In addition, the generic questionnaire SF-12 does not seem to capture the changes in HRQoL in these patients