OBJECTIVES: The recently developed Forgotten Joint Score - 12 (FJS-12) is increasingly being used in patients with hip or knee pathologies to assess joint awareness following surgical or conservative treatment. Joint awareness is a construct considered to be especially relevant in patients with good to excellent outcome. In our study we aimed at investigating reliability, validity and sensitivity to change over time of the English version of the FJS-12 in patients with total knee arthroplasty (TKA) or total hip arthroplasty (THA). METHODS: Patients undergoing THA or TKA at the Royal Infirmary of Edinburgh were consecutively included in the study and assessed with the FJS-12 at pre-surgery, 6 months and 12 months. To determine convergent validity we administered the Oxford Hip Score (OHS) and the Oxford Knee Score (OKS). We calculated Cronbach's Alpha as a measure of internal consistency of the FJS-12. Sensitivity to change is given as effect size (Cohen's d). RESULTS: At present we have recruited 219 TKA patients and 193 THA patients (recruitment ongoing). Mean age of the TKA patients was 68.3 (SD 9.3) and 54.1% were female. Mean age in the THA group was 67.6 years (SD 10.5) and 116 (60.1%) were female. Cronbach's alpha was very high in both groups (0.95 in TKA patients and 0.96 in THA patients). Correlations with the OHS (r=0.76) and the OKS (r=0.72) indicated good convergent validity of the FJS-12. Effect sizes for change between 6 months and 12 months in FJS-12 scores were 0.40 in TKA patients and d=0.17 in THA patients. CONCLUSIONS: The English version of the FJS-12 showed high internal consistency in THA and TKA patients and good convergent validity with the OHS and OKS. Our findings suggest that the FJS-12 is a reliable and valid instrument for the assessment of outcome after THA and TKA.