This study will examine the abnormal immune response in patients with sporadic inclusion body myositis (s-IBM)-the most common inflammatory muscle disease in people over the age of 50. s-IBM progresses steadily and may lead to severe weakness and wasting of arm and leg muscles. Patients may become unable to perform daily living activities and be confined to wheelchairs. s-IBM is thought to be an autoimmune disease, in which the body's own immune system attacks healthy muscles. This study will explore the causes of the muscle tissue inflammation that is responsible for destruction of muscle fibers and weakness in this disease. Information from the study may help in the development of an effective treatment for this disease.