Idiopathic bronchiolitis obliterans organizing pneumonia (idiopathic BOOP) is a clinicopathological entity of unknown origin first described by Epler and colleagues in 1985. We report our findings from a study of two Japanese boys aged 12 and 13 years with idiopathic BOOP. The patients showed no respiratory symptoms nor signs of infection. There was no history of exposure to noxious agents or medications in either case. Laboratory data, including hematological, biochemical, and immunological tests, were normal, and pulmonary function tests showed only a slight nonprogressive decrease from normal predicted values in vital capacity and diffusing capacity. Serial radiographs in both cases showed migration of patchy infiltrates. Fiberoptic bronchoscopy was performed for evaluation of eosinophilic pneumonia, but the bronchoalveolar lavage (BAL) revealed no increase in eosinophils and a decrease in CD4/CD8 ratio in the BAL fluid. Histopathological findings of transbronchial lung biopsy (TBLB) specimens were compatible with a diagnosis of BOOP. As neither case has shown clinical abnormalities apart from migratory patchy infiltrates on chest radiographs, both patients have been observed without drug therapy for 3-4 years.