Various adverse events can occur during antineoplastic therapy. A 67-year-old diabetic woman developed an emphysematous urinary tract infection (UTI) associated with chemoradiotherapy for lung cancer. She had received weekly carboplatin plus paclitaxel with thoracic radiotherapy and developed a fever on day 19. Computed tomography showed a large quantity of gas within the urinary tract. She was therefore diagnosed with emphysematous UTI. Poor diabetes control due to the weekly administration of dexamethasone, an existing urinary tract obstruction, and bone marrow suppression were involved in her serious infection. The potential development of emphysematous UTI during chemoradiotherapy should be considered in at-risk patients.