During recent decades, pulmonologists have tackled adult patients with fever, malaise, anorexia/weight loss, dry non-productive cough, progressively mild dyspnea and patchy infiltrates, which have not responded to antibiotic therapy. These patients might become diagnosed as having cryptogenic organizing pneumonia (COP), an inflammatory lung disease entity with presence of distinct constellation of clinical, radiological, and pathological features and absence of a known cause or a peculiar clinical context. Histopathological diagnosis is generally required and the morphology includes presence of intraalveolar buds of the myxomatous and fibrous connective tissue (organizing pneumonia) that have spread into the bronchioles (bronchiolitis obliterans) and, to a lesser extent, inflammation that has penetrated in the tissues surrounding the terminal conducting airways. COP is included in the heterogeneous family of idiopathic interstitial pneumonias (IIPs) because of its idiopathic nature, similarity with other IIPs, and presence of interstitial inflammation. Apart from COP, which is an idiopathic disease, organizing pneumonia (OP) is a well-known condition that develops within a specific context such as connective tissue diseases, inflammatory bowel disease, inhalation injury, various haematological disorders and malignancies, and secondary to pulmonary infections, as well as iatrogenic agents and actions such as lung transplantation, bone marrow grafting, radiation therapy, and use of certain medicines causing drug-induced lung disease. As far as there is no inflammation caused by microorganisms, COP does not respond to antibiotics, but a dramatic clinical and radiographic improvement is usually achieved with use of systemic corticosteroids. However, relapses are not uncommon after stopping treatment.