Methods: In our study we have investigated the prevalence of BRCA1 founder mutations: 300T/G, 4153delA and 5382insC in a population-based series of unselected breast (n= 2546) and ovarian (n= 795) cancer cases from Latvia. Among 4153delA and 5382insC mutation carriers identified in this analysis we made a comparison of overall survival, age at diagnosis and family histories of breast and ovarian cancers. Results: The prevalence of BRCA1 5382insC mutation carriers among breast cancer patients was significantly higher than 4153delA mutation carriers (Odds Ratio= 2.76, 95% CI= 1.74 to 4.38, P< 0.0001), whereas among ovarian cancer cases the prevalence of carriers of both these founder mutations was almost similar. We have also observed the difference in prevalence of breast and ovarian cancer cases among 1st and 2nd degree relatives of BRCA1 4153delA and 5382insC mutation carriers who were reported by the probands to have had any cancer. In addition among breast cancer patients BRCA1 mutation 4153delA was connected to later age of onset and to worse clinical outcomes in comparison with BRCA1 5382insC mutation.Conclusion: Based on our data we can suggest that carriers of BRCA1 4153delA and 5382insC founder mutations have different risk of breast and ovarian cancer development, different age of onset and prognosis of breast cancers. These results indicate the importance of further clinical studies for evaluation of the significance of different cancer prevention options among carriers of specific BRCA1 mutations.