['Fetal cells have been demonstrated in the active lesions of adult women with systemic sclerosis. Because the juvenile idiopathic inflammatory myopathies (JIIM) share clinical and histopathological features with systemic sclerosis and graft-vs-host disease, we explored the possibility that maternal cells persist and play a role in the pathogenesis of JIIM.', 'DNA samples extracted from peripheral blood of 28 JIIM patients (14 females, 14 males) and 23 healthy controls were assessed for microchimerism by the HLA Cw polymerase chain reaction method. HLA Cw alleles from eight mothers and three healthy siblings of JIIM patients were also examined.', 'A microchimeric allele was identified in 19 of 26 JIIM patients whose data were able to be interpreted, compared with two of 21 healthy controls (P<0.001). Subjects with microchimerism ranged in age from 4 to 28 yr. In eight cases in which maternal peripheral blood was available, the additional Cw allele present in the patients was confirmed to be identical to a maternal allele. Three healthy siblings of JIIM patients did not have evidence of a microchimeric Cw allele.'] Is persistent maternally derived peripheral microchimerism associated with the juvenile idiopathic inflammatory myopathies?