The previously held concept that the fetus is completely separated from the mother, especially by trophoblasts that line the outer layer of the placenta, has recently been questioned. It has recently been shown that fetal cells are detectable not only in the peripheral blood, but also in maternal skin and liver. Although the migration of fetal cells into the maternal circulation has been given a great deal of attention because of its implication in the prenatal diagnosis of genetic diseases, the potential role of such placental transfer of fetal cells in the pathogenesis of autoimmune diseases has only recently been considered. In patients with scleroderma, fetal cell-derived DNA was detected more frequently in the peripheral blood of patients than controls. This finding of a limited number of fetal cells in maternal tissues leading to microchimerism has been proposed to have a role in the induction of scleroderma. Although evidence for microchimerism has also been confirmed in a high frequency of liver tissues from patients with primary biliary cirrhosis (PBC), a similar high frequency was noted in control patients, which suggests that microchimerism by itself is unlikely to fully account for the pathogenesis of PBC. The finding of such a high frequency of fetal microchimerism in the liver suggests that this is a very common event, raising the possibility that the migration of fetal cells may be important in the induction and subsequent maintenance of tolerance against the fetus during pregnancy. In addition, it is clearly possible that such microchimerism could contribute to the pathogenesis of select autoimmune diseases.