When an acute pneumonia does not resolve, it progresses to a subacute and chronic stage, with organization of the acute fibrinous exudates by fibroblast ingrowth. An organized acute pneumonia consists of consolidation of the lung, with fibroblastic proliferation (granulation tissue) filling the airspaces. This early fibrosis may be admixed with inflammatory cells, residual acute pneumonia, and aggregates of macrophages in a patchy pattern.

A pattern of lung injury previously referred to as bronchiolitis obliterans organizing pneumonia consists of plugs of granulation tissue with bronchiolar lumens and in adjoining airspaces. This particular pattern is now more often designated organizing pneumonia and, when idiopathic, cryptogenic organizing pneumonia. These patterns are discussed in Chapters 62 and 94, respectively.