The right atrium and the right ventricle of fifty-five patients were imaged with transesophageal echocardiography during fifty-nine total knee arthroplasties performed with cement and the use of general anesthesia. The patients ranged in age from thirty-two to eighty-three years (mean, 65.5 years). Cardiopulmonary parameters were measured with use of hemodynamic monitoring systems, such as pulse oximeters, pulmonary artery catheters, and radial artery catheters. In addition, a femoral vein catheter was inserted on the side of the operation in ten of the fifty-five patients. Showers of echogenic material traversing the right atrium, the right ventricle, and the pulmonary artery after the tourniquet was deflated were observed to various degrees in all patients and lasted three to fifteen minutes. The mean peak intensity occurred within thirty seconds (range, twenty-four to forty-five seconds) after the tourniquet was released. The mean mixed venous oxygen saturation (and standard error of the mean) decreased (from 83 ± 0.9 to 72 ± 1.5 per cent) and the mean pulmonary arterial pressure increased (from 20 ± 1.0 to 27 ± 1.0 millimeters of mercury [2.67 ± 0.13 to 3.60 ± 0.13 kilopascals]), compared with the values before the tourniquet was released, in all patients. The pulmonary vascular resistance index increased after release of the tourniquet (to a maximum of 328 ± 29 dyne·s·cm-5·m2; p = 0.00002) only in the patients who had echogenic material that was at least 0.5 centimeter in diameter. Clinical pulmonary embolism developed postoperatively in three patients; all three had had echogenic particles that were more than 0.5 centimeter in maximum diameter on imaging. Blood aspirated from one of the pulmonary artery catheters and from five of the ten femoral vein catheters demonstrated fresh venous thrombus. Histological evaluation of the aspirates failed to demonstrate fat, marrow, or particles of polymethylmethacrylate.Surgeons should consider acute pulmonary embolism as a diagnosis when evaluating a patient who has hemodynamic collapse during total knee arthroplasty performed with cement.