Lung cancer is the leading cause of cancer-related death among all types of cancers. In lung cancer patients, hospitalization rates in intensive care units (ICUs) are gradually increasing due to both illness- and treatment-related complications. A 61-year-old male patient who had undergone radiotherapy for small cell lung cancer (SCLC) was admitted to the ICU on the 3rd day of his treatment because of hypercapnic respiratory failure. High-flow oxygen therapy was started. The mass lesion showed rapid progression. Chemotherapy was performed for SCLC which is a chemosensitive tumor.