To facilitate the positional cloning of the breast-ovarian cancer gene BRCA1, we constructed a high-density genetic map of the 8.3-cM interval between D17S250 and GIP on chromosome 17q12-q21. Markers were mapped by linkage in the CEPH and in extended kindreds in our breast cancer series. The map comprises 33 ordered polymorphisms, including 12 genes and 21 anonymous markers, yielding an average of one polymorphism every 250 kb. Twenty-five of the markers are PCR-based systems. The order of polymorphic genes and markers is cen-D17S250-D17S518-HER2-THRA1-RARA-D17S80 -KRT10-[D17S800-D17S857]-GAS- D17S856-EDH17B-D17S855-D17S859-D17S858-[++ +PPY-D17S78]-D17S183-EPB3-D17S579- D17S509-[D17S508-D17S190 = D17S810]-D17S791-[D17S181 = D17S806]-D17S797- HOX2B-GP3A-[D17S507 = GIP]-qter. BRCA1 lies in the middle of the interval, between THRA1 and D17S183. Markers from this map can be used to determine whether cancer is linked to BRCA1 in families, to evaluate whether tumors have lost heterozygosity at loci in the region, and to identify probes for characterizing chromosomal rearrangements from patients and from tumors.