Bronchiolitis obliterans organizing pneumonia (BOOP) is a clinicopathological syndrome associated with a variety of disease entities. The aim of this study was to review cases with initial diagnosis of BOOP applying uniform histopathologic criteria, and analyze the clinical characteristics of proven cases of BOOP including rapidly progressive form. A total of 81 cases, initially diagnosed as BOOP and with available tissue sections, was collected. Thirty six cases (44.4%) were excluded from the study, more than two thirds of which were given a revised diagnosis of interstitial pneumonitis/fibrosis other than BOOP. Thirty one patients were classified as idiopathic BOOP, 8 patients as secondary BOOP, and 6 patients as rapidly progressive BOOP. Open lung biopsy specimen from all six cases with lethal outcome showed more severe interstitial inflammation and septal fibrosis and/or alveolar exudate with a varying degree than those with good prognosis. There was no difference by the sexes. The two most frequent presenting symptoms were cough and dyspnea. Bilateral multifocal consolidation was a common radiological finding. More than 70% cases of idiopathic BOOP experienced clinical improvements. The diagnosis of BOOP is usually suggested by clinicoradiologic findings, but needs to be confirmed histopathologically, preferably through surgical open or video-assisted thoracoscopic biopsy.