Background: Myelosuppression, usually manifested as neutropenia, anemia, and/or thrombocytopenia, is a common complication of chemotherapy for extensive-stage small cell lung cancer (ES-SCLC). Myelosuppression has a substantial impact on both the patients and the healthcare system. Despite the published literature on risk factors associated with neutropenia among cancer patients, there is very limited research on whether there are risk factors associated with myelosuppression among ES-SCLC. This study examined the association between patient attributes and the risk of myelosuppression in ES-SCLC, utilizing real-world data from US community oncology practices. Methods: A retrospective observational study analyzed US Oncology Network iKnowMed electronic health records data from 1/1/2015 to 12/31/2020. The study identified adult ES-SCLC patients who began chemotherapy between 1/1/2015 and 12/31/2019 and followed them until 12/31/2020, the date of their last visit or death, whichever occurred earliest. Myelosuppression events were identified based on Common Terminology Criteria for Adverse Events v5.0 definitions for anemia, neutropenia, and thrombocytopenia. Multivariate regression analyses were performed to examine the association between patient characteristics and the risk of experiencing grade ≥3 myelosuppression in any lineages. Sensitivity analyses were conducted on risk of grade ≥3 myelosuppression in each lineage. Results: A total of 1,574 adult patients with ES-SCLC receiving chemotherapy were included (mean age 68 years, 82.2% White, 47.6% male, and 55.5% with an ECOG score ≤1). During follow-up (mean 8.9 months), 56.6% of patients had grade ≥3 myelosuppression. Patient demographics (age, sex, and race) were not identified as factors associated with having grade ≥3 myelosuppression in any lineages. Prophylactic G-CSF use was associated with lower risk of grade ≥3 myelosuppression (OR=0.61, p<0.0001). Receiving a higher dose of chemotherapy was associated with increased risk for grade ≥3 myelosuppressive events (OR=1.14 for 1 mg of increase in dose of carboplatin, p=0.03 and OR=1.01 for 1 mg increase in cisplatin, p=0.003). Having an index treatment hold was associated with higher risk for grade ≥3 myelosuppressive events (OR=2.16, p<0.001). Lastly, having an index treatment delay was associated with higher risk for grade ≥3 myelosuppressive events (OR=1.68, p<0.001). The area under the ROC curve was 0.64 for the main model and ranged 0.60-0.70 in the sensitivity analyses. Conclusions: Patient characteristics were not significant risk factors for myelosuppressive events in ES-SCLC, while chemotherapy intensity and treatment patterns played a more prominent role. The study suggests that prophylactic management may help reduce the risk of myelosuppression in ES-SCLC patients.