Large series of case reports of juvenile polymyositis/dermatomyositis (JPM/JDM) are now seldom found in either Chinese or overseas literature. Clinical and pathological data of 42 cases of JPM/DM, including serum enzymes, electromyography (EMG) and muscular pathology were studied retrospectively. Patients with JPM/JDM manifested muscle weakness, muscle atrophy, elevation of creatine kinase and abnormal EMG with myogenic damage. Children with JDM generally manifested characteristic cutaneous abnormalities. Subcutaneous calcifications were present in some chronic cases of JDM. Muscle biopsy demonstrated immune inflammation in both JPM and JDM cases. The incidence of endomysium inflammatory in patients with JPM was higher than that in patients with JDM ( 70.8% vs 38.9% ) (P< 0.05 ), whereas the incidences of microvasculopathy ( 54.2% vs 94.4% ; P< 0.01 ) and perifascicular atrophy ( 41.7% vs 72.2% ; P< 0.05 ) was lower in JPM compared to JDM. Infiltrates were more frequently seen in JPM. Corticosteroid therapy was effective in most JPM/DM patients. It was concluded that there are some clinical and pathological differences between JPM and JDM. JPM frequently manifests sporadic muscle fibre degeneration, necrosis and endomysium inflammatory, while microvasculopathy and perifascicular atrophy are the characteristic pathology of JDM.