Abstract Breast cancer is the leading cause of cancer incidence and mortality among Hispanic women, who are often diagnosed with late stage tumors and are more likely to die after diagnosis when compared to non-Hispanic whites. While strides have been made in understanding Hispanic breast cancer genetics, most studies have been limited to investigating cases from high-risk clinics. In this study, we aimed to assess the prevalence of pathogenic mutations in three most important breast cancer genes (BRCA1, BRCA2 and PALB2) in an unselected Hispanic breast cancer cohort. Initially, six hundred and forty-six unselected Colombian breast cancer cases were screened for four known Hispanic BRCA1/2 mutations by genotyping. Subsequently, cases that remained mutation negative, as well as 186 cancer-free controls, were screened for BRCA1, BRCA2 and PALB2 mutations using targeted next generation sequencing. We identified 67 cases with pathogenic mutations, one case with a likely-pathogenic variant, and 16 carriers of variants of unknown significance. Among the pathogenic mutation carriers, 88.1% harbored a founder mutation (n = 59), which includes the known BRCA1 3450del4 mutation, strikingly found in 32 unrelated cases. Remarkably, we found that 1 in 4 of the cases diagnosed with breast cancer by age 40 years, regardless of family history of cancer, carried a pathogenic mutation. The high frequency of pathogenic mutations in this unselected cohort (10.4%), in particular those with an early age of onset (25.3%), suggests that population-based genetic testing among Hispanic communities can identify most carriers who would otherwise ineligible for testing. Identifying mutation carriers of these genes has implications in clinical management and surveillance for Hispanic women, a population that is vulnerable to breast cancer disparities in the U.S. and Latin America. Citation Format: Anna Marie Tuazon, Mabel Bohorquez, Carolina Ramirez, Paul Lott, Ana Estrada, Angel Criollo, Cathy Wang, Magdalena Echeverry, John Suarez, COLUMBUS Consortium, LaFamilia Consortium, Luis G. Carvajal Carmona. One in four Hispanic women with early onset breast cancer carry BRCA1, BRCA2, or PALB2 mutations: Results from a population-based study in South America. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1786.