Middle lobe syndrome is defined as recurrent or chronic collapse of the middle lobe of the right lung. It occurs in all age groups and is divided into an obstructive type, with a demonstrable airway occlusion, and a nonobstructive type, with a patent right middle lobe bronchus apparent on bronchoscopy. Middle lobe collapse has specific radiographic findings. Malignancy is the most common cause of the obstructive type, and infections are the second leading etiology. The nonobstructive type is associated with inflammatory conditions and bronchiectasis. In all cases, treatment is directed at the underlying cause.