Purpose ; Endobronchial tuberculosis and bronchogenic cancer are common causes of atelectasis or obstructivepneumonitis in Korea. Differntiation between endobronchial tuberculosis and bronchogenic carcinoma is importantfor the treatment and prognosis but it is sometimes difficult to differentiate these two lesions with radiologicexaminations. The purpose of this study was to find the differential points between endobronchial tuberculosis andbronchogenic carcinoma associated with atelectasis or obstructive pneumonitis. Materials and Methods ; Fortypatients in whom atelectasis or obstructive pneumonitis was detected on chest radiographs comprised the study. Adefinite mass opacity was not observed on chest radiographs in all patients. In these patients, the causes ofobstruction were endobronchial tuberculosis (n=20) and bronchogenic cancer (n=20) which were microbiologically orpathologically confirmed. Results ; Double obstructive lesions were more frequently found in endobronchialtuberculosis (8/20) than in bronchogenic cancer (1/20). Multiple calcifications along the bronchial wall andsevere distortion of bronchi were observed only in endobronchial tuberculosis (4/20) and associated low densitymass at obstruction site was only observed in bronchogenic cancer (6/20). Bronchial dilatation (11/20) andparenchymal calcifications (14/20) distal to obstruction site, air containing bronchogram at post obstructivebronchus (14/20) were more frequently found in endobronchial tuberculosis. Contour bulging at obstruction site(14/20), and only mucus bronchogram at post obstructive bronchus (14/20) are more frequently found in bronchogeniccarcinoma. Conculusion : In patients with atelectasis or obstructive pneumonitis, endobronchial tuberculosis ischaracterized by double obstructive lesion, multiple calcifications at the bronchial wall, and severe distortionof the bronchi. Endobronchial carcinoma is characterized by a low density mass at the obstructive site.