Total knee arthroplasty (TKA) without a tourniquet can reduce the risk of large venous emboli, but is associated with increased blood loss. To provide an accurate estimate of this blood loss, we divided 136 patients who underwent primary TKA into four groups based on the type of operation: 1) bilateral TKA with tourniquet; 2) bilateral TKA without tourniquet; 3) unilateral TKA with tourniquet; and 4) unilateral TKA without tourniquet. Blood loss was estimated by calculating the fall in hemoglobin and total blood volume corrected for Japanese. A small difference in intraoperative blood loss was seen between unilateral TKA without tourniquet (308 ml) and bilateral TKA without tourniquet (411 ml). Total blood loss was 879 ml in unilateral TKA with tourniquet, 1165 ml in unilateral TKA without tourniquet, 1458 ml in bilateral TKA with tourniquet and 1823 ml in bilateral TKA without tourniquet. Uni- and bilateral TKAs without a tourniquet were thus associated with an increased blood loss. Performing TKA without a tourniquet should thus be reconsidered to avoid venous thromboembolism.