The paradoxical movement of the mediastinum, in relation to the respiratory cycle, is a valuable fluoroscopie sign in the evaluation of partial obstruction of air-flow through the trachea or larynx. Its recognition in the presence of the normal chest radiograms together with the clinical history of aspiration, is of importance in the diagnosis of a foregin body located above the tracheal bifurcation. The pathophysiology and the radiographic manifestations of this phenomenon are described. Various etiologies are discussed in which this sign is also seen. The experience with 18 pediatric patients is presented and 3 cases are outlined in details.