In a prospective study of eighty press-fit condylar knee arthroplasties performed with cement, we compared the postoperative blood loss in forty patients in whom the defect left by the femoral intramedullary alignment rod had been left open with the blood loss in forty patients in whom the defect had been closed with a plug of cement. The patients in the open-defect group had significantly more blood loss (mean, 1002 milliliters) in the first twenty-four hours after the operation than the closed-defect group (mean, 752 milliliters) (p less-than-or-equal-to 0.01). The total postoperative blood loss was also significantly greater in the open-defect group (mean, 1536 milliliters) than in the closed-defect group (mean, 1215 milliliters) (p = 0.02).