A substantial drop in blood volume occurs in patients being treated by total knee arthroplasty (TKA). Of 140 TKAs (108 patients) studied to analyze this blood loss, 70 required transfusion and 70 did not. The average transfusion was 2.6 units per arthroplasty. Blood loss in the nontransfused group was 1.8 units per arthroplasty. The overall mean blood loss was 2.2 units per TKA. Insertion of a constrained TKA resulted in a statistically significant increase in blood loss. Preoperative diagnosis, anesthetic technique, revision arthroplasty, patellofemoral arthroplasty, and tourniquet technique did not statistically affect the blood loss. The bulk of the blood loss is collected postoperatively in the suction drainage system.