Systemic sclerosis (SSc) is a disease of unknown origin. Although SSc is considered to be an autoimmune disease, recent studies have implicated cellular microchimerism in its pathogenesis. Microchimerism results from the persistence of fetal cells, from prior pregnancies, in the maternal circulation. The demonstration of the presence of fetal CD3+ T cells in the maternal circulation and of fetal cells in affected SSc tissues suggests that microchimerism might cause SSc in certain patients by initiating a graft-versus-host-like response.