The hereditary breast cancer gene BRCA1 previously has been localized to chromosome 17q21. We looked for evidence of involvement of this region of chromosome 17 in 130 sporadic breast cancers. Seventeen polymorphic sequence tagged site markers were examined in these tumors between the D17S250 and D17S579 loci to screen for deletions as measured by loss of heterozygosity. The smallest common region that was deleted occurred in the approximately 120-kilobase interval between the D17S846 and D17S746 loci within the BRCA1 region. Delineation of this commonly deleted area should accelerate attempts to identify the involved gene(s) and its relationship to BRCA1.