Idiopathic bronchiolitis obliterans with organizing pneumonia (BOOP) is a specific clinicopathologic syndrome characterized by a pneumonia-like illness, with excessive proliferation of granulation tissue within bronchioles, alveolar ducts and alveoli. The changes are most numerous in alveolar ducts. The presence of intraluminal tufts of organizing connective tissue in alveolar ducts and more distal airspaces has been termed organizing pneumonia The radiologic manifestations are distinctive with bilateral, diffuse alveolar opacities predominantly in the subpleural and lower lung zone. Patchy migratory pneumonic foci or solely nodular infiltrates are rarely present in BOOP. BOOP is a diagnosis of importance because of its dramatic response to steroids.