There are several bronchiolar diseases with different pathologic and clinical findings. Idiopathic BOOP is a distinct entity consisting of a flu-like illness, late inspiratory crackles, patchy infiltrates roentgenographically, and physiologically decreased vital capacity and diffusing capacity. Response to corticosteroid therapy is good and relapse does not occur if sufficient therapy is given. Bronchiolitis obliterans organizing pneumonia is an appropriate description of this entity. The term is specific because it includes bronchioles and alveoli and excludes disorders involving only alveoli such as organizing pneumonitis or organizing diffuse alveolar damage. The term is general enough to include a sufficient number of patients with a homogenous disorder. Furthermore, the entity can be described to clinicians and pathologists throughout the world in such a fashion that patient care and research can be standardized. Researchers from different centers studying the cause or utilizing treatment protocols are able to discuss a single BOOP entity rather than comparing results of a heterogeneous group of many different types of interstitial lung disorders. This will lead to breakthroughs in discovery of etiologic causes and new effective therapeutic regimens.