When bronchogenic carcinoma is coexisting with pulmonary tuberculosis, it is difficult to differentiate bronchogenic carcinoma from pulmonary tuberculosis radiologically. Thus, the object of this study is to define differential diagnosis of bronichogenic carcinoma by computed tomography. We analized CT scans of 27 patients with radiologic findings of pulmonary tuberculosis and mass of which twelve cases were pulmonary tuberculosis and fifteen cases were primary lung cancer. The location of parenchymal infiltration and the mass was the same in 60% (9/15) of the primary lung cancer in cases and 83%(10/12) of the pulmonary tuberculosis cases. The common location of the mass was the both upper lobes in 92%(11/12) of the pulmonary tuberculosis cases and 53%(8/15) of the primary lung cancer cases. The common locations of the mediastinal lymphadenopathy were 4R, 2R of the pulmonary tuberculosis cases and 4R, 1OR of the primary lung cancer cases. In the feature of post enhanced lymph odes, homogenous increased density was more frequent in primary lung cancer. Measurements of the maximum thickness part of the cavity wall was not a reliable indication of malignancy.