In total knee arthroplasty (TKA) the total blood loss is composed of 'visible' blood loss from the surgical field and wound drainage, and blood loss into the tissues which is hidden. Blood management should be aimed at addressing the total blood loss. The measured loss is generally an underestimation of the 'true' loss, which can be calculated. Sixty-three TKAs were prospectively studied at the Avon Orthopaedic Centre, Bristol, UK. All but two TKAs were performed under a tourniquet. Thus the intraoperative blood loss was usually zero. Reinfusion of drained blood was carried out when appropriate. The drainage and volume reinfused were recorded. None of the patients studied received a bank blood transfusion. All patients had FBC including haematocrit (Hct) pre- and post-operatively. Each patients height and weight were recorded pre-operatively. The true total blood loss was calculated using height, weight and pre- and post-op Hct. Subtracting the visible loss gives the hidden loss. As the object of the study was to determine the proportion of blood loss that is hidden, rather than document blood loss in general, seven patients with very large blood losses were excluded from the study to minimise the effect of haemodilution on our calculation. The threshold chosen was 1.5 l. In TKA, the mean total calculated blood loss was 1474 ml. The mean hidden loss was 735 ml. It can be seen that the hidden loss is 50% of the total loss, making the true loss following TKA twice the volume usually recorded. This is due to bleeding into the tissues and haemolysis when reinfusion is carried out. It is concluded that when planning blood management after TKA, account should be taken of the probable substantial hidden blood loss.