Juvenile dermatomyositis is a disease with similarities to chronic graft-versus-host disease. To identify whether chimerism is present in juvenile dermatomyositis, we investigated the families of 15 children with the disorder. Chimerism was identified by PCR in 13 of the 15 affected children, compared with five of 35 siblings (p<0.0001). Maternal cells among peripheral-blood mononuclear cells were detected in 11 of the 15 boys, compared with five of 17 unaffected controls (p=0.02), and in muscle tissue of 12 of 15 compared with two of ten unaffected siblings (p=0.005). These results suggest that chimerism may be involved in juvenile dermatomyositis.