The anterior junction line is a normal anatomic landmark on chest radiographs of healthy adults and older children caused by the visceral and parietal pleurae of the two lungs meeting anteriorly at the midline. It is not seen on chest radiographs of healthy infants. When this sign is identified on the supine frontal view of a neonate, it signifies bilateral pneumothorax. In this situation the line is formed by the meeting of the medial parietal pleurae on each side as they herniate anteriorly in front of the thymus and heart. The sign is best seen when the patient is rotated slightly to the left. It is not seen when there is unilateral pneumothorax or a concomitant pneumomediastinum. Although the anterior junction line is not a highly sensitive indicator of bilateral pneumothorax, it is highly specific and its recognition can promote faster diagnosis.