Our objective was to ascertain whether displacement of the aortic knob on chest radiographs could be used as a sign to detect a left paratracheal esophageal mass. Sixty-one consecutive pathologically proven esophageal cancer patients were included in this study according to the following criteria: tumor at the aortic arch level; chest radiographs in the neutral position; no unilateral volume loss in the lung. Sixty-one sex- and age-matched subjects served as the control group. To measure the extent of displacement of the aortic arch, we drew a circle over the aortic knob, fitting more than one-quarter of the circumference on the chest radiograph. The distance between the medial end of the circle and the left margin of the trachea (aortic displacement value, ADV) was measured. The difference of the ADV between the study group and the control group was analyzed using a paired t test. The aortic displacement value was significantly larger in the study group (11.7+/-4.5 mm) than in the control group (5.6+/-2.9 mm). When we applied 10 mm as a threshold level, sensitivity and specificity on detection of esophageal cancer were 78.7 and 78.7%, respectively. Displacement of the aortic knob may be a useful sign to indicate a left paratracheal esophageal mass.