['Although hypertension has long been recognized as a factor that might increase intraoperative blood losses in major orthopedic surgery, the effects of postoperative pain-induced hypertension on blood losses have not so far been evaluated. The aim of this study was to evaluate the effect of pain on perioperative blood losses of patients undergoing primary total knee replacement (TKR).', 'Data from patients participating in a randomized clinical trial comparing intravenous patient-controlled analgesia (PCA) (N = 20) with PCA plus continuous femoral nerve (three-in-one) block (N = 20) or PCA plus continuous posterior lumbar plexus (psoas compartment) block (N = 20) were prospectively and retrospectively collected. Correlations between relevant variables and measured and calculated blood loss, number of transfused unit, and late (96 hours) postoperative hemoglobin were tested by linear regressions. Stepwise regressions for each of the four above-mentioned goals were constructed using a probability to enter of 0.25 and to leave of 0.1. A P < 0.05 was considered significant.', 'At the stepwise regressions there was a significant positive correlation between measured blood losses and morphine consumption from 12 to 18 hours (P = 0.006); between calculated blood loss and preoperative mean arterial blood pressure (P = 0.01) and preoperative hemoglobin value (P = 0.02); and between late postoperative hemoglobin and body weight (P = 0.047).'] Does postoperative pain significantly influence postoperative blood loss in patients undergoing total knee replacement?