Fetal microchimerism (FMc) describes long-term persistence of small numbers of fetal-derived allogeneic cells in the mother. Although FMc has been implicated as a mechanism of autoimmune disease, it may confer a beneficial effect with immune surveillance of malignant cells. We hypothesized that allogeneic FMc imparts a protective effect against breast cancer. Two observations provided a rationale for the study hypothesis. First, allogeneic cells convey risk reduction for recurrent malignancy in hematopoietic cell transplantation. Second, reduced risk of breast cancer is well recognized among parous compared with nulliparous women. As an initial test of the hypothesis, we investigated 82 women, 35 with breast cancer and 47 who were healthy, for male DNA in peripheral blood, presumed from a prior pregnancy with a male fetus. The prevalence and levels of male DNA were determined by real-time quantitative PCR for the Y chromosome-specific gene DYS14 in DNA extracted from peripheral blood mononuclear cells. FMc was found significantly more often in healthy women than women with breast cancer (43% versus 14%, respectively). Considering the absence of FMc as a risk factor, the odds ratio was 4.4 [95% confidence intervals (95% CI), 1.34-16.99; P = 0.006]. Restricting analysis to women known to had given birth to a son, the odds ratio was 5.9 (95% CI, 1.26-6.69; P = 0.01). Our findings indicate that allogeneic FMc may contribute to reduction in risk of breast cancer. Further studies are indicated and, if confirmed, extended studies to examine whether allogeneic immune surveillance from FMc is deficient in women with breast cancer.