Two cases of idiopathic bronchiolitis obliterans organizing pneumonia (BOOP) with unusual CT findings are presented. On CT both cases exhibited crescentic and ring-shaped opacities, surrounding areas of groundglass attenuation, and associated with a nodular pattern in one patient and airspace consolidations in the second patient. CT-pathologic correlation disclosed that the central areas of groundglass attenuation corresponded to alveolar septal inflammation, in contrast to the denser periphery where granulomatous tissue in peripheral airspaces predominated. In the broad spectrum of CT findings, BOOP can exhibit specific CT features with regard to the crescentic or ring-shaped opacities with a central groundglass attenuation area. Since these features have not been described in any other disease, they might be characteristic features for the diagnosis of BOOP.