The majority of, but not all, women with mutations in the BRCA1 gene will be affected with breast or ovarian cancer by the age of 70. To establish whether known risk factors modify susceptibility to cancer in these women, we have studied the reproductive histories of 333 North American women who were found by haplotype analysis to carry BRCA1 mutations. An increased risk for breast cancer was associated with low parity and with recent birth cohort. The risk of ovarian cancer decreased with increasing age at last childbirth; however, in contrast to the case for sporadic cancer, the risk of ovarian cancer in BRCA1 carriers was found to increase significantly with increasing parity.