Haplotype-sharing was examined in sets of affected siblings in the Breast Cancer Linkage Consortium pedigrees [Easton et al., 1993], using both identity-by-descent and identity-by-state methods. Linkage of the disease susceptibility locus to markers on chromosome 17 was confirmed. Substantial genetic heterogeneity was found, with more young cases being attributable to the putative gene. No differences were found between breast and ovarian cancer, and there was no evidence against a simple dominant mode of inheritance. © 1995 Wiley-Liss, Inc.