Obliterative bronchiolitis is a histologic diagnosis of dense eosinophilic fibrous submucosal plaques in the membranous and respiratory bronchioles. It results from repeated injury, acute rejection, infection, and other inflammatory reactions. These features lead to airway epithelial damage, followed by an exaggerated healing response. There is a build-up of mesenchymal cells and collagen fibers in the lumen which results in fibrosis and partial or total obliteration of the airways. The role of immunologic factors in this process is discussed.