The radiographic appearance of retrothymic pneumomediastinum is quite specific. The findings include elevation of the thymus away from other mediastinal structures with increased lucency beneath it, visualization of a radiodense line extending from the inferior pole of the thymus to the midportion of the heart, and tenting of the pericardium at the point of attachment of this line. This constellation of findings will aid in the differential diagnosis of medial anterior pneumothorax, pneumopericardium, and intrapulmonary cyst in the infant with air block. The paper details the mediastinal anatomy with special emphasis on a previously undescribed layer of connective tissue that accounts for this specific radiographic appearance.