At least 25% of Ashkenazi Jewish families with two or more cases of premenopausal breast cancers are attributable to one of three founder mutations in BRCA1 or BRCA2. As these three founder mutations are common in the Ashkenazi Jewish population ( approximately 2.5%) and can easily be tested for in a multiplex assay, establishing ethnicity can expedite genetic testing. It is not always possible, however, to conclusively establish ethnicity before offering testing. We report here the occurrence of a founder Ashkenazi Jewish BRCA1 mutation, 185delAG (also known as 187delAG), in a non-Jewish Chilean family with no reported Jewish ancestry. The linked haplotype present in this family was identical to that identified in the Ashkenazi Jewish population. This case report not only illustrates the problem of the definition of ethnicity but also points to the possibility of further studies of the frequency of founder Ashkenazi Jewish mutations in populations not generally considered to be of Ashkenazi Jewish origin.