Background: Cyclin-dependent kinase 4 & 6 inhibitors (CDK 4 & 6i) have changed the treatment (trt) landscape for hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced/metastatic breast cancer (ABC) patients (pts). This study examined real-world pt characteristics, select adverse events (AEs), and related outcomes of these pts. Methods: Female pts in the United States Oncology Network’s iKnowMedTM electronic health records (EHR) database were eligible if diagnosed with HR+/HER2- ABC and initiated CDK 4 & 6i across any line between 1 JAN 2017 and 31 DEC 2018. Index date was initiation of CDK 4 & 6i, and baseline period was 6 months prior to index. Structured and unstructured EHR data were abstracted. Analyses were descriptive by design; baseline characteristics were not adjusted across sub-cohorts or by follow-up time, therefore outcome analyses are not comparative. Results: The average pt age was 64 (SD 12) years (396 total, 163 palbociclib, 142 abemaciclib, and 91 ribociclib). Among those with baseline performance status (n=307), 268 (87%) pts had an Eastern Cooperative Oncology Group performance status of 0-1. The most common metastatic sites at index were bone (74%), visceral metastases (45%; defined as liver, lung, pleural, peritoneal, or adrenal gland involvement), lymph nodes (23%), lung (22%), and liver (20%). Relative to pts receiving palbociclib or ribociclib, a greater percent of abemaciclib pts had visceral metastases; fewer had bone or bone-only metastases (p<0.05). The most frequent pre-specified comorbidities at index were diabetes mellitus (16.7%), chronic obstructive lung disease (8%), venous thromboembolism (6%), and renal disease (5%). Pts initiated CDK 4 & 6i in the first- (69%), second- (22%), third- (6%), and fourth- or later lines (3%), and the percentages were similar by line for each therapy. Overall, 61% of pts discontinued index CDK 4 & 6i; time to discontinuation of the index trt will be reported. Reasons for discontinuation included progression (25%), toxicity (20%), and hospice (4%). Frequency of AEs attributed to the index CDK 4 & 6i are shown in Table 1. Outcomes of AEs overall include trt hold (25.8%), dose reduction (25.5%), trt discontinued (16.9%), hospitalization (3.5%), and emergency room visit (5.1%). Conclusions: This descriptive analysis provides insights into pt characteristics, real-world AEs, and related outcomes with CDK 4 & 6i within a US community oncology network.