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Simple Summary: Palbociclib combined with an aromatase inhibitor (AI) has been shown to be effective in clinical trials for people with HR+/HER2− breast cancer that has spread to other areas of the body, such as the lungs or liver. Evidence of palbociclib effectiveness in routine clinical practice can provide complementary support for clinical trial findings. This study used electronic health records of people with breast cancer that had spread to their lungs and/or liver to determine how well palbociclib plus an AI worked compared to an AI alone. The study showed that palbociclib plus an AI compared with an AI alone was associated with a 38% or 27% reduction in the risk of death for patients with breast cancer that had spread to the lungs or liver, respectively. These findings support the use of palbociclib plus an AI for people whose breast cancer has spread to their lungs or liver. A cyclin-dependent kinase 4/6 inhibitor combined with endocrine therapy is the standard of care for patients with hormone receptor-positive/human epidermal growth factor 2-negative (HR+/HER2−) metastatic breast cancer (mBC), but real-world effectiveness data for patients with lung or liver metastases are limited. This retrospective study included data from the US Flatiron Health database of patients with HR+/HER2− mBC and lung or liver metastases treated with first-line palbociclib (PAL) plus an aromatase inhibitor (AI) or an AI alone in routine clinical practice. Overall survival (OS) and real-world progression-free survival (rwPFS) were assessed. A total of 891 patients were included (622 with lung metastasis, 376 with liver metastasis, and 107 with both lung and liver metastasis). After stabilized inverse probability of treatment weighting to balance patient characteristics, PAL + AI versus AI alone was associated with significantly prolonged OS (HR = 0.62; p < 0.001) and rwPFS (HR = 0.55; p < 0.001) in patients with lung metastases and numerically longer OS (HR = 0.73; p = 0.056) and significantly longer rwPFS (HR = 0.57, p < 0.001) for those with liver metastases. Overall, PAL + AI versus AI alone was associated with prolonged OS and rwPFS in routine clinical practice, supporting the use of first-line PAL + AI for patients with HR+/HER2− mBC with lung and/or liver metastases. [ABSTRACT FROM AUTHOR]