Background: The exact contribution of TP53 germline mutations, associated with Li Fraumeni Syndrome, to the overall burden of cancer is still only partially known. Studies in Southern and Southeastern Brazil have shown that a specific germline mutation at codon 337 (c.1010G>A; p.R337H), has incomplete penetrance and may be present in a significant number of subjects (estimated frequency at the populational level of 1:300 individuals). In an exploratory approach, the aim of the study is to assess the frequency of the p.R337H mutation in women from different Brazilian regions, diagnosed with breast cancer (BC) before 46 and after 55 years of age, and unselected for family history of cancer. Methods: Formalin-fixed paraffin-embedded (FFPE) non-tumoral tissue (lymph nodes or normal breast) of women diagnosed with BC between 2000 and 2010 in 3 pathology laboratories from the Brazilian cities of Porto Alegre, São Paulo and Barretos were obtained retrospectively and analyzed after anonimization. Genomic DNA was isolated with standard methods and genotyping performed in duplicates by qPCR (TaqMan assay). Confirmation of all mutation-positive and a sample of mutation-negative cases were done by TP53 exon 10 sequencing or by a second independent qPCR analysis. Results: Analysis of 515 BC-affected women identified the p.R337H mutation in the germline of 70 (8,6%) cases: 49/403 (12,1%) diagnosed before 46 years and 21/412 (5,1%) diagnosed after 55 years. BC occurred earlier in p.R337H mutation carriers than in non-carriers (p=0.001). Conclusions: Preliminary analysis in a sample of women with BC indicates that p.R337H founder mutation is present in a high proportion of cases, especially those diagnosed at a young age. Regional genetic background and recruitment strategies may account for the different mutation frequencies observed in the centers of study. The occurrence of this mutation at such a high frequency in a particular geographic region has important implications for disease management and cancer risk counseling for these patients and families. This mutation likely contributes to a significant proportion of the health burden associated with BC in Brazil. Financial support: FIPE-HCPA, CAPES, FAPERGS and Glaxo Smith Kline.