1563 Background: Limited data exist on the real-world treatment patterns and effectiveness of cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors in the germline BRCA (g BRCA) mutated breast cancer population. Methods: Adults with human epidermal growth factor receptor 2 negative (HER2-), hormone receptor positive (HR+) metastatic breast cancer (mBC) treated with a CDK4/6 inhibitor from 01Jan2013–31Jan2018 were retrospectively selected from the Flatiron Health Oncology electronic medical record database. Patients with known g BRCA status were classified as having g BRCA mutated (g BRCAm) or wild type (g BRCAwt) disease. Time to first subsequent therapy or death (TFST) and overall survival (OS) were calculated from start of the earliest line of therapy with a CDK4/6 inhibitor. Kaplan-Meier (KM) medians were estimated, and TFST and OS compared between g BRCA groups with Cox models stratified by line of therapy and adjusting for demographic and clinical characteristics that modified hazard ratios (HRs) for g BRCA status by > 10%. Results: Of 2968 HER2- HR+ patients with mBC receiving a CDK4/6 inhibitor, g BRCA status was known for 859 (28.9%). Patients with g BRCAm and g BRCAwt received letrozole plus palbociclib (42.4 and 39.8%, respectively), fulvestrant plus palbociclib (32.9 and 30.7%), or other CDK4/6 regimens (24.7 and 29.5%) across all lines. The g BRCAm group had a non-significant, shorter TFST than g BRCAwt (stratified HR 1.24; 95% CI 0.96–1.59). OS was significantly shorter in g BRCAm than g BRCAwt patients (stratified HR 1.50; 95% CI 1.06–2.14). Conclusions: The results of this real-world study suggest that treatment outcomes with CDK4/6 inhibitors may be poorer in patients with g BRCAm compared with g BRCAwt disease. These findings indicate a higher unmet need among patients with g BRCAm, potentially requiring alternative treatment options. [Table: see text]