We collected data from all patients treated with first-line atezolizumab, etoposide, and carboplatin from the tumor registry at Pusan National University Hospital, Busan, South Korea (atezolizumab group) from April 2020. Furthermore, we analyzed the data obtained from a cohort of patients with ES-SCLC treated with chemotherapy alone (chemo-only group) between January 2018 and March 2020. Eligible patients included those who had been diagnosed with ES-SCLC in accordance with the Veterans Administration Lung Cancer Study Group. Patients with previous definitive concurrent chemoradiation therapy for limited-stage SCLC were also excluded. The study protocol was approved by the Institutional Review Board (IRB) of Pusan National University Hospital (IRB no. 2208-016-118). Moreover, the study was conducted in accordance with the principles of the Declaration of Helsinki. The requirement for informed consent was waived by the IRB of Pusan National University Hospital because of the retrospective nature of the study, and the analysis used anonymous clinical data.