Mediastinal pseudomass: pneumonia and atelectasis behind the left pulmonary ligament.

In children, left lower lobe pneumonia posterior to the pulmonary ligament may mimic a paramediastinal mass. Over a period of 6 years we have seen 12 children with this unusual appearance, which we attribute to a peculiar type of atelectasis or infiltration of the left lower lobe or a segment of it. The radiographic appearance is thought to be the result of incomplete anchoring of the left lung by a short pulmonary ligament. The radiographic findings are demonstrated, and the benign clinical course is emphasized.