Juvenile dermatomyositis is a relatively rare, multisystem disease characterized by a nonsuppurative myositis which causes symmetrical weakness, rash and vasculitis; this last can affect the gastrointestinal tract and the myocardium. Late development of calcinosis is seen in approximately two thirds of patients. Its etiology is unknown, although there are clues that it may be an unusual response to a viral infection. Some 50% of children will have a very acute, rapidly progressive disease, while the remainder may present subacutely with rash and a gradually progressive weakness of muscles, joint contractures and very occasionally calcinosis. When there is acute muscle damage, the creatine phosphokinase will be raised, but it is not uncommon to have a normal erythrocyte sedimentation rate, and antinuclear antibodies are usually present. Early in acute cases immune complexes will often be detected. In the presence of vasculitis, monitoring the disease by levels of von Willebrand's factor 8 antigen may be helpful. Although the prognosis for survival has steadily improved, it remains a serious illness and death can occur in the acute phase due to myocarditis, progressive unresponsive myositis, perforation of the bowel as a sequel to vasculitis ulceration or occasionally lung involvement. Intercurrent infections during the course of the disease also give rise to problems. In its management, there is still a question as to whether intravenous pulses of methylprednisone might be more valuable than oral corticosteroids; in either case it must be given in adequate amounts early in the course of the disease to control muscle inflammation. Once this is controlled rehabilitation commences.(ABSTRACT TRUNCATED AT 250 WORDS)