Fleischner lines are often the only roentgen evidence of pulmonary embolization, particularly when the emboli are small. The linear shadows are cast by focal areas of atelectasis viewed on end. The atelectasis is an indirect manifestation of the vascular occlusion and is due to obstruction of a subsegmental bronchus by retained secretion or by hemorrhagic exudate. Although Fleischner lines are not seen in every case of pulmonary embolization, when present, and especially when bilateral, they are definitely suggestive of the diagnosis.