The right lateral decubitus position is a risk factor for postoperative pulmonary embolism. We examined postural changes of femoral vein velocity in order to elucidate the mechanism. Thirty patients scheduled for general thoracic surgery were enrolled in this study. The common femoral veins on both sides were examined by color-duplex ultrasound for venous caliber and velocity when the patients were in the right lateral, left lateral, and supine positions. The maximum diameters of the right femoral vein in the right lateral decubitus position and the left femoral vein in the left decubitus position were significantly larger than those in the other positions. The venous velocity of the right femoral vein in the right lateral decubitus position was significantly smaller than that in the supine position, while the velocity of the left femoral vein in the left lateral decubitus position was not significantly decreased. We speculate that the decreased venous velocity of the right femoral vein in the right lateral decubitus position could result in a deep venous thromboembolism in the right leg, making this position a possible risk factor for postoperative pulmonary embolism.