The disease concept of idiopathic BOOP has emerged from a study of many open lung biopsy cases of diffuse infiltrative lung disease. The histopathologic features of idiopathic BOOP have several components: bronchiolitis obliterans, organizing pneumonia, accumulation of foamy cells in the peripheral air spaces, and interstitial infiltration of mononuclear cells. These pathologic findings are nonspecific and many conditions show such a BOOP pattern. Idiopathic BOOP has been discussed in the context of bronchiolitis obliterans, organizing pneumonia, and interstitial pneumonia. While clinically idiopathic BOOP has a relatively broad spectrum of manifestation, BOOP stands as a clinicopathologic disease entity among diffuse infiltrative lung diseases of unknown etiology.