Objectives. To analyse the differences in blood requirements and length of stay in patients undergoing total knee arthroplasty during the period before and after the implementation of the fast track protocol. Material and methods. Retrospective observational follow-up study or cohorts. The variables under study were: initial and minimum haemoglobin, number of patients and units transfused and days of hospital stay. For the comparisons, the chi-square and U-Mann Whitney statistics were calculated. Values of p?0.05 were considered significant. Results. A sample of 189 patients was studied. The pre- and post-implementation values for the variables under study were: minimum haemoglobin 10.2 vs. 11.2 (p = 0.0), percentage of patients who required blood transfusion 20.8 vs. 4.5% (p = 0.001), rate of anaemia 3.4 vs. 2.9 (p = 0.002) and lengths of hospital stay 5.9 vs. 3.8 days (p = 0.017). Conclusions. The fast track protocol significantly improved haemoglobin values and decreased the need for transfusion and length of stay