Background: Small cell lung cancer (SCLC) is an aggressive type of lung cancer. Most patients (pts) are diagnosed with extensive-stage disease (ES-SCLC). The cornerstone treatment of ES-SCLC has been platinum-based chemotherapy and etoposide. Combining atezolizumab with carboplatin plus etoposide (Carbo-E) improved overall survival (OS) in ES-SCLC pts and became a new standard for first-line treatment. However, there is a paucity of real-world outcomes in ES-SCLC. Herein, we present the largest and longest follow-up retrospective study analyzing atezolizumab in combination with chemotherapy for treatment of ES-SCLC. Methods: We conducted a retrospective study and included all adult pts (≥ 18 years) with ES-SCLC treated at Cleveland Clinic between 1/2010-12/2022. ES-SCLC was defined as stage IIIB or stage IV. We collected treatment regimens and baseline characteristics including age, sex, race, smoking, alcohol intake history, and comorbidities. Responses were assessed using RECIST response criteria. Both OS and progression-free survival (PFS) were calculated from treatment initiation. We used propensity score (PS) weighting and multivariable Cox proportional hazards ratio (CPH) to adjust for baseline confounding. Results: We identified 561 ES-SCLC pts who received treatment, 375 (67%) received Carbo-E and 160 (29%) received Carbo-E + Atezolizumab (Carbo-E-Atezo). The table describes the baseline characteristics of Carbo-E and Carbo-E-Atezo groups. The median follow-up was 30 months (mo) (IQR: 14 - 59). In the Carbo-E arm, 45 patients (12%) received second-line immune therapy (nivolumab +/- ipilimumab or pembrolizumab). On PS-adjusted logistic regression, pts who received Carbo-E-Atezo had a 1.1 odds ratio (95%CI: 0.85-1.43, P>0.05) to respond compared to pts who received Carbo-E. There was no difference in PS-adjusted median OS between pts who received Carbo-E-Atezo (9.2 mo (95%CI: 8.1-11)) vs. Carbo-E (8.4 mo (95%CI: 7.7-9.0)) (log-rank P>0.05). Similarly, there was no difference in adjusted median PFS between pts who received Carbo-E-Atezo (5.9 mo (95%CI: 5.3-6.9)) vs. Carbo-E (5.5 mo (95%CI: 4.7-5.9)) (P>0.05). Using CPH model, the addition of atezolizumab to Carbo-E did not decrease mortality (Hazard Ratio: 0.84, 95%CI: 0.68-1.04, P>0.05). Male gender and increasing age were predictors of worse OS (P<0.05). Conclusions: Among pts with ES-SCLC, the addition of atezolizumab to the standard chemotherapy regimen of Carbo-E showed no significant difference in PFS or OS in our real-world study.