AIM: To identify factors significant in influencing LOS following primary TKA in a UK specialist arthroplasty centre. METHOD: We retrospectively reviewed factors affecting LOS of 514 patients who underwent primary TKA in a single specialist arthroplasty centre in the UK over a one-year period. Surgical and patient factors that may influence LOS were recorded. RESULTS: The median LOS was 5days. 85% were discharged within 10days. Those staying longer were classified as long stay (up to 3months). The only surgical factor that influenced length of stay was postoperative blood transfusion (p35 (p=0.009) and age>80 (p=0.01). CONCLUSION: Length of stay is largely due to case mix and this should be considered when planning a local arthroplasty service. LEVEL OF EVIDENCE: 2-II.