We previously identified a polymorphism in the human estrogen receptor gene. In a preliminary study on women with estrogen receptor positive (ER+) breast tumors, we found that the presence of the rarer of the two alleles, the B' allele, is correlated with a history of spontaneous abortion. Because that study evaluated only women with estrogen receptor positive (ER+) breast cancer, it was unknown whether the observed correlation was restricted to the cancer group or was independent of breast cancer. We have now extended our analysis to include not only additional women with ER+ breast cancer, but also those with estrogen receptor negative (ER-) breast cancer and women without cancer. Results of the current study continue to show an association between the B' allele and a history of spontaneous abortion in the ER+ breast cancer group. There was no such correlation either in the ER- breast cancer group or in the group without cancer. Also, we continue to observe, in the ER+ breast cancer group, a significantly higher concentration of ER protein in tumors from homozygous wild type women (genotype BB), than in the tumors from women who are heterozygous for the rarer allele (genotype BB'). We conclude that the combination of spontaneous abortion and the BB' ER genotype may be a marker for breast cancer susceptibility.