Bronchiolitis obliterans organizing pneumonia (BOOP) is increasingly recognized as an important cause of diffuse infiltrative lung disease. It is a diagnostic consideration in patients with a febrile flu-like illness of a few weeks' duration and a roentgenogram showing bilateral patchy infiltrates that are not responsive to a typical course of antibiotics. It is defined as granulated tissue plugs within lumens of small airways that extend into alveolar ducts and alveoli. Clinically, a flu-like illness, cough, and crackles are common. Pulmonary function studies of patients show a decreased vital capacity, normal flow rates (except in smokers), and a decreased diffusing capacity. It is generally idiopathic, but it may occur during the resolution of a viral or mycoplasma pneumonia. It is also associated with a variety of systemic illnesses and clinical settings. These include the connective tissue disorders, antineoplastic and other drugs, and immunological disorders, as well as bone marrow and lung transplantation. There are numerous related disorders, including human immunodeficiency virus infection, radiation therapy, thyroiditis, and alcoholic cirrhosis. In idiopathic BOOP, complete resolution occurs in 65% to 85% of patients treated with corticosteroid therapy. This type of therapy is often effective in patients with associated systemic disorders or in other clinical settings, but there may be limited or no response in patients with dermatomyositis, immunosuppression, or interstitial opacities at the lung bases. Respiratory failure leading to death may occur in 5% of patients. It is important to add BOOP to the differential diagnosis of febrile, noninfectious illnesses that are mimics of pneumonia.