Growth of oat cell carcinoma tends to be invasive and extends rapidly through the bronchial lymphatics to the hilus and mediastinum, where bulky mass of tumor develop. Authors have analysed roentgenologic manifestations of 22 cases of histologically proven oat cell carcinoma of the lung seen during the period of 3 years from jan. 1980 to May.1983. The results were as follows. 1. They were 18 males and 4 females. incidence was the most common in 7th decade as 45%. 2. Chief complaints are cough, sputum and dyspnea. Metastatic symptoms are hoarseness, SVC syndrome and back pain. 3. The radiographic findings of oat cell carcinoma were as follows. 1) Hilar and perihilar mass 73% 2) Mediastinal mass 64% 3) Bronchial obstruction sign 55% 4) Peripheral mass 18% 5) Pleural effusion 18%