The high frequency of aggressive, early onset, and highly fatal breast cancer among Chesapeake Bay region African Americans suggests that there may be a contributing ancestral component. This study identifies the region's founding African, European, and Native American Indian populations using ethnogenetic layering and identifies the microethnic substructure of each founding continental aggregate. The largest component (38%) of the enslaved Africans brought to the Chesapeake Bay originally came from the coastal and hinterlands of the Bight of Bonny, a region with very high rates of aggressive, early onset breast cancer. Ethnogenetic layering is applied a second time to reveal the microethnic groups of the Bight of Bonny hinterlands with historical links to the Chesapeake Bay region. These analyses identify the specific microethnic groups within this region of Africa that may be the sources of relevant polymorphisms contributing to the etiology of aggressive breast cancer in the Chesapeake Bay. This report suggests a historical link between specific African microethnic groups and a US health disparity. Am. J. Hum. Biol., 2008. © 2008 Wiley-Liss, Inc.