Organizing pneumonia was previously referred to as bronchiolitis obliterans organizing pneumonia, or BOOP. It may occur in organizing acute bacterial infections, viral infections, or other infections, as a reaction to fumes and toxins, in postobstructive lung, with aspiration, in drug reactions, as a result of chemotherapy or radiation therapy, and as a component of specific lung diseases including hypersensitivity pneumonitis, collagen vascular diseases, eosinophilic pneumonia, and Wegener granulomatosis (Table 18-1). It may also be seen as an idiopathic clinical syndrome called cryptogenic organizing pneumonia (COP), previously recognized as idiopathic BOOP. Transbronchial biopsies show lung parenchyma with scattered foci or “plugs” of granulation tissue in the lumens of alveoli. If sampled on the transbronchial biopsy, the granulation tissue may also be seen in alveolar ducts and/or bronchioles. The granulation tissue consists of edematous, myxoid stroma, often with a bluish or grayish tinge on hematoxylin and eosin (H&E), and variable numbers of fibroblasts and sometimes capillaries. The granulation tissue foci or “plugs” may be of variable size and relatively inconspicuous at low power in some biopsies. There may be additional findings of interstitial pneumonia, often lymphocytes, and occasionally specific findings that indicate the cause of the organizing pneumonia such as viral inclusions, fungal organisms, aspirated food or material, infectious granulomas, poorly formed granulomas of hypersensitivity pneumonitis, numerous eosinophils, and so on. Intra-alveolar foamy macrophages suggest small airway obstruction and may be seen in association with organizing pneumonia or may be the only indication of small airway obstruction in a clinically suspected case of COP or BOOP.