The objective of this study was to examine the characteristics of hidden blood loss and assess the effects of using a tourniquet on postoperative hidden loss in patients undergoing primary total knee arthroplasty. Eighty patients were randomised into two groups: one group underwent operation with a tourniquet and one without. Operating time, perioperative blood loss, hidden blood loss, free haemoglobin,
swelling, ecchymosis, straight leg raising action and knee flexion were measured. There were significant differences in the hidden blood loss, free haemoglobin, postoperative swelling, extent of ecchymosis, straight leg raising and postoperative knee flexion in the early period after operation between the two groups. Our results indicate that knee arthroplasty operations with a tourniquet might promote postoperative hidden blood loss and hinder patients' in early postoperative rehabilitation exercises.