Mediastinal lung herniation is a rare condition characterized by protrusion of 1 lower lung through behind the heart into the opposite side of the chest, usually from right to left. We present a case of mediastinal lung herniation associated with pulmonary sequestration, which was confirmed both surgically and pathologically in a 13-year-old girl initially admitted with a diagnosis of pneumonia. Contrast-enhanced computed tomographic images using a multidetector-row computed tomography clearly demonstrated the right lung herniation toward the left and 2 aberrant systemic arteries supplying the sequestered lung mass. These arteries run through the herniated lung from right to left. Additionally, on the basis of pleural anatomy, we discuss herein the difference between a mediastinal lung herniation and horseshoe lung.