The conviction that has rapidly forced itself upon clinicians is that COP is a clinicopathological syndrome relevant in pulmonary medicine. Obscured for a long time by both ambiguities in the clinical interpretation of the pathologic term "bronchiolitis obliterans" and the enduring concept that organizing pneumonia was almost always merely of usual infectious origin, it finally was characterized when clinical and pathologic data were carefully correlated using the clinicopathologic method advocated by Laënnec at the beginning of the last century. COP is now easily recognized in its typical form, in which patients present with a subacute inflammatory pulmonary disease with patchy alveolar opacities on chest imaging. A negative etiologic investigation and characteristic pathologic features confirm the diagnosis. Corticosteroids are a highly effective treatment, despite the relapses occurring when therapy is stopped too rapidly, further indicating that COP is an inflammatory process that may persist for prolonged periods. Less typical forms of COP, solitary focal and diffuse infiltrative COP, need further characterization to define their respective limits with typical COP and UIP. Whatever its origin(s), COP clearly merits recognition as a distinct entity on clinical, imaging, pathologic, and evolutive grounds. COP probably is not a new disease, but its delayed recognition proves that clinicopathologic studies are still of interest.