Patients 18 years of age and older with active dermatomyositis or polymyositis that does not respond adequately to treatment with methotrexate and corticosteroids may be eligible for this study. Candidates will be screened with a medical history, physical examination, blood and urine tests, chest x-ray, pulmonary function test, skin test for tuberculosis, HIV test, electromyography (described below), manual muscle testing, and functional assessments. Magnetic resonance imaging (described below) will be done to assess the degree and location of inflammation in the involved limbs. An electrocardiogram and echocardiogram will be done if recent ones are not available. Patients who qualify for the study will be asked to undergo two muscle biopsies (surgical removal and analysis of small pieces of muscle tissue), one before initiation of treatment and another on the 16th week .