We used data from a population-based case-control study to evaluate Janerich's hypothesis that reduced risk of breast cancer in women with multiple marriages may be attributable to an immune response to fetal or sperm antigens. Risk of breast cancer in women with multiple marriages was reduced relative to that in women who had been married only once; however, there was no indication that such risk was reduced among women whose full-term pregnancies were fathered by different men, relative to women whose pregnancies were each fathered by the same man. Increasing lifetime number of male sexual partners was associated with a trend of decreasing risk of breast cancer. Our results indicate that, if there are effects of exposure to fetal or male antigens on risk of female breast cancer, their impact may be heterogeneous.