Trans-mediastinal herniation of lung is seen occasionally but herniation of bulla across the mediastinum is rare. We report two cases with trans-mediastinal herniation of bullae leading to an unusual line appearing near the mediastinum. We propose the name 'Semilunar sign'for this radiological sign because of its resemblance to the half-moon shape.
Keywords: Bulla; chronic obstructive pulmonary disease; trans-mediastinal herniation.