A breast-ovarian cancer susceptibility gene, BRCA1, which is responsible for disease in approximately 45% of breast cancer families and most families that contain breast and ovarian cancer, has been assigned by genetic linkage to 17q12-21. Here, we report the analysis of three marker-disease recombinants in families that contain breast and ovarian cancer, two of which strongly suggest a location for BRCA1 telomeric to D17S702, a microsatellite polymorphism, and a third which suggests a location centromeric to EDH17B, the gene encoding estradiol-17B dehydrogenase. If the interpretation of these recombinants is correct, the results localise BRCA1 to an interval of < or = 1 cM.