Congenital lobar overinflation, often called congenital (or infantile) lobar emphysema, refers to marked enlargement of a lobe due to airspace enlargement. It most often affects the upper lobes, particularly the left upper lobe. It is an uncommon cause of respiratory distress in infancy and in only a very few cases has been detected by prenatal ultrasound. Affected infants usually become symptomatic within the first month of life, and presentation after 6 months of age is unusual. Although there may be partial regression in size in utero, lobectomy may be necessary because of excessive air trapping postnatally.

Congenital lobar overinflation is thought to be caused by partial obstruction, either intrinsic or extrinsic, to a lobar bronchus. Complete obstruction is rare but has been identified; obstruction is most often partial and may be due to bronchomalacia, mucus plugs, webs or stenoses, bronchial torsion, or extrinsic compression by vascular structures, bronchogenic cysts, or postinflammatory strictures. Defective or absent bronchial cartilage may also cause expiratory collapse and hyperinflation. The lobe is typically markedly enlarged but, in contrast to polyalveolar lobe, shows only airspace enlargement rather than increased numbers of alveoli. Polyalveolar lobe may be a variant of congenital lobar overinflation or may be a separate condition. It is much less common than congenital lobar overinflation.