A tracheal diverticulum is usually an incidental finding. It projects posteriorly where the cartilage rings are deficient and usually lies to the right where there is no esophagus supporting the paratracheal tissue. A direct connection with the trachea is often visible on CT. It may be congenital or acquired. The acquired form is thought to be due to prolonged increased intraluminal pressure such as with a chronic cough. Although usually asymptomatic, it may accumulate respiratory secretions that become infected and lead to cough or tracheobronchitis. The differential diagnosis of a paratracheal air cyst also includes an esophageal diverticulum and an apical bleb or bulla.