 It is estimated that 175,000 new breast cancer cases and 44,5000 cancer deaths will result from female breast cancer in 1991. One of the most important risk factors is a family history of breast cancer. A susceptibility allele which accounts for a large proportion of familial breast cancer has been localized to chromosome 17q and attempts to clone the susceptibility are proceeding rapidly. The existence of recognized susceptibility loci and the awareness of a significan proportion of genetic predisposition and the associated penetrances remain unknown and are impossible to estimate without bias from previous studies. This study is designed to estimate heterogeniety and prenetrance in a series of unslected families, identify additional breast cancer susceptibility locs, as well as provide resources for gene isolation and mutation analysis studies. 