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Background and Objectives: Palbociclib is a cyclin-dependent kinase 4/6 inhibitor that is approved in the United States for the treatment of hormone receptor‒positive (HR+)/human epidermal growth factor receptor‒2 negative (HER2−) advanced breast cancer (ABC). The objectives of this expanded access trial were to provide palbociclib in combination with letrozole to patients with HR+/HER2− ABC in Argentina, Brazil, Colombia, and Mexico who were candidates for letrozole therapy before commercial availability of palbociclib, and to evaluate the safety and tolerability of palbociclib plus letrozole. Patients and Methods: Postmenopausal women aged ≥ 18 years with HR+/HER2− ABC were eligible to participate in this study. Patients received palbociclib 125 mg once daily (3/1 schedule) and letrozole 2.5 mg once daily (continuous schedule). Safety, objective response rate (ORR), and duration of treatment were evaluated. Results: A total of 130 patients were treated with palbociclib plus letrozole (Argentina, n = 33; Brazil, n = 35; Colombia, n = 28; Mexico, n = 34). The most common treatment-emergent adverse events (TEAEs) of any grade were neutropenia (70.0%), leukopenia (34.6%), anemia (33.8%), decreased neutrophil count (27.7%), and thrombocytopenia (24.6%); 22.3% of patients required a palbociclib dose reduction due to adverse events (AEs). Serious AEs were reported in 32 patients (24.6%). The ORR was 24.8% (95% confidence interval 17.6‒33.2), and the median duration of treatment was 10.6 months (range 0.1‒29.3). Conclusion: Palbociclib in combination with letrozole was generally well tolerated with a clinically manageable safety profile; the observed ORR supported treatment benefit in Latin American women with HR+/HER2− ABC. Trial Registry: ClinicalTrials.gov, NCT02600923. Plain Language Summary: This study was done to learn more about the safety of 2 medicines together for women with advanced breast cancer after menopause. All 130 women in the study had the most common kind of breast cancer and were from Argentina, Brazil, Colombia, and Mexico. Everyone took 2 oral medicines called palbociclib and letrozole during the study. The researchers looked for any side effects experienced by the women while taking these medicines together. Another goal of the study was to see how well the treatment worked. Blood tests showed 70.0% of women had a side effect where they had a lower number of a type of white blood cell called a neutrophil. In total, 34.6% of women had low levels of another white blood cell called a leukocyte. These blood test results can mean a person is more likely to get infections. Serious side effects were experienced by 24.6% of the women, which meant these were life-threatening, caused lasting problems, or they needed hospital care. To cope with their side effects, 22.3% of the women switched to a lower palbociclib dose; 24.8% of the women had an overall response, which meant they either had a decrease in their tumor size or all cancer signs disappeared from their body. The most common length of time in the study was 10.6 months and the longest time was 29.3 months. The results of this study support using palbociclib plus letrozole to treat women who live in Latin America with advanced breast cancer after menopause. [ABSTRACT FROM AUTHOR]