Computed tomography provides excellent visualization of the trachea, but there have been only limited descriptions of variations in tracheal shape. The normal tracheal cartilage is U shaped. Lunate shaped trachea is uncommon and is characterized by a coronal/sagittal diameter ratio (tracheal index) greater than 1. We present the CT appearance of a lunate shaped trachea with corresponding pathologic sections. Destruction of the membranous septum caused marked increase in the tracheal index in this patient.