Idiopathic bronchiolitis obliterans organizing pneumonia (BOOP) and cryptogenic organizing pneumonia (COP) are synonyms for an inflammatory interstitial process characterized by young fibromyxoid connective tissue within airways and air spaces. This clinicopathologic condition is associated with an excellent response to steroidal therapy in more than 80% of patients. In this study, we examined matched groups of 10 patients with steroid-responsive idiopathic BOOP/COP and 9 patients with idiopathic BOOP/COP who fared poorly despite therapy. No significant differences in demographics, symptoms, or radiographic appearances were noted between the two cohorts. Histologic examination revealed that the cases with progressive, idiopathic BOOP/COP were accompanied by scarring and remodeling of the background lung parenchyma in 89% of cases, in contrast to 10% of those with a good prognosis. This finding provided a potential morphologic marker of outcome for therapy in idiopathic BOOP/COP. Steroid-nonresponsive cases of BOOP/COP may have a propensity to cause irreversible injury to the lung, a feature not seen in cases with a good outcome. Another explanation may be that such cases represent a BOOP/COP-like reaction pattern in patients with an associated fibrosing interstitial pneumonia, especially usual interstitial pneumonia. The differential diagnosis of BOOP/COP with organizing diffuse alveolar damage, eosinophilic pneumonia, and other chronic interstitial pneumonias is discussed.