"Organizing" refers to unresolved pneumonia (in which the alveolar exudate persists and eventually undergoes fibrosis) in which fibrous tissue forms in the alveoli.

The American Thoracic Society and the European Respiratory Society hold that "cryptogenic organizing pneumonia" is the preferred clinical term for this disease for multiple reasons:[6][7] The classic presentation of COP is the development of nonspecific systemic (e.g., fevers, chills, night sweats, fatigue, weight loss) and respiratory (e.g. difficulty breathing, cough) symptoms in association with filling of the lung alveoli that is visible on chest x-ray.

[8] This presentation is usually so suggestive of an infection that the majority of patients with COP have been treated with at least one failed course of antibiotics by the time the true diagnosis is made.

[9] The fibroblasts differentiate into myofibroblasts and continue to form fibrosis resulting in intra-alveolar fibroinflammatory buds (Masson's Bodies) that are characteristic of organizing pneumonia.

[9] Remodeling occurs, resulting in the intra-alveolar fibroinflammatory buds (Masson's Bodies) moving into the interstitial space and forming collagen globules that are then covered by type 1 alveolar epithelial cells with well developed basement membranes.

Laboratory findings are nonspecific but inflammatory markers such as the erythrocyte sedimentation rate or C-reactive protein and the lymphocyte count are frequently elevated.

[9] Resolution of inflammatory cells in the bronchoalveolar lavage is usually delayed in organizing pneumonia, lagging behind clinical and radiographic improvement.

[9] Biopsy findings in patients with organizing pneumonia consist of loose connective tissue plugs involving the alveoli, alveolar ducts and bronchioles.

[9] The chest x-ray is distinctive with features that appear similar to an extensive pneumonia, with both lungs showing widespread white patches.

On high resolution computed tomography, airspace consolidation with air bronchograms is present in more than 90% of patients, often with a lower zone predominance.

One rare presentation, focal organizing pneumonia, may be indistinguishable from lung cancer based on imaging alone, requiring biopsy or surgical resection to make the diagnosis.