Fetal microchimerism is the presence of fetal cells inmaternal tissues and vice versa, i.e., the coexistence of2 different cellular populations from genetically differentindividuals within a single person. The most frequentcause of microchimerism is pregnancy, in which there is abi-directional fetal-maternal interchange of cells duringpregnancy and delivery. Fetal cells have been demonstrated in the tissues ofpatients with rheumatic, endocrine or infectious diseases,as well as in those of healthy individuals. Microchimerism has been most extensively studied insystemic sclerosis. It seems that during pregnancyallogenic fetal or maternal cells cross the placenta bidirectionallyand persist in the systemic circulation andtissues of both mother and child. Subsequently, they areactivated, resulting in is a graft-against-host reactionassociated with the onset of clinical manifestations.Microchimerism has been also studied in otherconnective tissue diseases.