The persistence of fetal stem cells with multilineage potential in women who have been pregnant, a phenomenon known as fetal microchimerism, is emerging as a potential contributing factor in certain diseases, including cancer. For example, fetal microchimerism has been implicated in autoimmune disease, wound healing, and cancer. Studies of this phenomenon may provide a novel perspective on cancer in women, including in breast, ovarian, and lung cancers. [Cancer Res 2008;68(23):9567-9]