A 70-year-old woman complained of cough, sputum and fever. Chest roentgenogram showed a mass-like lesion with pleural effusion in the left thorax. Bronchoscopy revealed a polypoid lesion at the orifice of left B10 and mucosal swelling of the left lower lobe bronchi. The biopsied material of the polypoid lesion was diagnosed as malignant lymphoma (diffuse small cell type by LSG classification) and the immunohistochemical examination showed monoclonal proliferation of T lymphocytes. The atypical lymphocytes which were positive for T lymphocyte surface marker were detected in the pleural effusion. There was no evidence of metastasis in any part of the body under clinical examinations, and the lung was thought to be a primary site of the lymphoma. The doubling time of tumor was estimated to be 134 days. Four-drug combination chemotherapy resulted in a marked decrease of the tumor size. Immunohistochemical study to determine the subpopulation of lymphoma cells are required for diagnosis of pulmonary lymphoma, because the biological characters of T cell lymphoma might differ from those of B cell lymphoma.