We described three children with juvenile dermatomyositis (JDM) refractory to the conventional therapy. They were successfully treated with intravenous cyclophosphamide (IVCY) pulses, and two of them were administered plasma exchange (PE) before IVCY. Case 1. A 17-year-old girl with JDM was previously treated for 2 years with the combination of prednisolone, intravenous gamma-globulin, methotrexate, and azathioprine. However, muscle weakness gradually progressed. She failed to hold her sitting position and to rise her arms, but both serum CK and aldolase were stable. After the episode of aspiration pneumonia the follow-up muscle biopsy was performed, which revealed muscle degeneration and massive mononuclear cell infiltration in perivascular area. The erythrocyte sedimentation rate (ESR) and fibrin degradation product E (FDP-E) levels were gradually increased. Because the active inflammation of muscle and muscle vasculature was suspected, the PE and IVCY combination therapy was administered. During the 6 courses of the therapy, muscle weakness was markedly improved so that she could hold herself at the sitting position and could have meals by herself. Case 2. A 5-year-old boy with JDM was treated for 8 months with prednisolone p.o., but his muscle strength became worse. The muscle enzyme levels, such as serum CK and aldolase, were not reflecting his status of the disease, but FDP-E levels were increased. Muscle MRI and biopsy revealed the inflammatory changes of perivascular area of muscle. The PE and IVCY combination therapy was effective, and he became able to walk and run by himself. Case 3. A 14-year-old boy was diagnosed as having JDM when he was 10 years of age, and treated with prednisolone p.o., and subsequently with intravenous methylprednisolone pulses and azathioprine. Three years later the flares were observed accompanied with the elevations of serum CK and FDP-E. The administration of IVCY improved muscle strength as well as serum muscle enzyme and FDP-E levels. These findings indicated that the clinical manifestations of JDM should be closely monitored, that the serum levels of muscle enzymes including CK and aldolase were sometimes not indicative for the flares of JDM, and that muscle MRI and re-biopsy examination were needed for the children with progressive muscle weakness. In addition, determination of ESR and FDP-E was not specific but helpful to detect flares of the disease in some cases.