Pulmonary blebs are small subpleural air-filled circumscribed cystic spaces, usually less than 1 cm in diameter. There is an association with small airways disease. Rupture causes pneumothorax and is associated with tobacco smoking.

Blebs are thought to occur as a result of subpleural alveolar rupture, due to strain overload of the elastic fibers.

Pulmonary bullae are, like blebs, cystic air spaces that have an imperceptible wall (less than 1 mm). Bullae are typically larger but there is no size cut-off to distinguish bullae from blebs. Blebs may, over time, coalesce to form bullae.

The smallest blebs are not visible on imaging but can be detected in pathological specimens.