Constrictive bronchiolitis is a condition in which the bronchiolar lumens are severely narrowed or obliterated by submucosal scarring. Constrictive bronchiolitis is also called bronchiolitis obliterans and obliterative bronchiolitis. Constrictive bronchiolitis results from scarring caused by infections (especially viral infections that affect the bronchioles), collagen vascular diseases involving the lungs (particularly rheumatoid arthritis), drug reactions, exposures to fumes and toxins, and bone marrow transplant. Constrictive bronchiolitis is a major histopathologic finding in chronic lung transplant rejection (see Chapter 104). It may be a component of bronchiectasis, cystic fibrosis, or asthma. It is also seen in some rare conditions, as in inflammatory bowel disease with lung involvement (see Chapter 90) and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (see Chapter 14, Part 4). Idiopathic cases also occur.