Growth of oat cell carcinoma tends to be invasive and extends rapidly through the bronchial lymphatics to thehilus and mediastinum, where bulky mass of tumor develop. Authors have analysed roentgenologic manifestations of22 cases of histologically proven oat cell carcinoma of the lung seen during the period of 3 years from Jan. 1980to May. 1983. The results were as follows. 1. They were 18 males and 4 females. Incedence was the most common in7th decade as 45%. 2. Chief complaints ae cough, sputum and dyspnea. Metastatic symptoms are hoarseness, SVCsyndrome and back pain. 3. The radiograhic findings of oat cell carcinoma were as follows. 1) Hilar and perihilarmass 73% 2) Mediastinal mass 64% 3) Bronchial obstruction sign 55% 4) Peripheral mass 18% 5) Pleural effusion 18%