Dermatomyositis, recognized as an autoimmune disorder, occurs not only in adults but also in children. In this study, we evaluated lymphocyte subpopulations in the peripheral blood of 24 adult dermatomyositis and 14 juvenile dermatomyositis patients and in 17 healthy adults and 9 healthy children by flow cytometry using monoclonal antibodies. When compared with healthy adults and adult patients with inactive dermatomyositis, the adult patients with active disease had significantly lower percentages of CD3+ and CD8+ cells and a significantly higher percentage of CD20+ cells. In contrast, juvenile dermatomyositis patients had lymphocyte subpopulations not significantly different from those of healthy children; the activity/inactivity of disease did not make any difference. These results support our hypothesis that adult and juvenile dermatomyositis may be diseases of entirely different scope.