By creating a registry, physicians will have the opportunity to understand the clinical outcomes of Myositis patients treated with Acthar. Despite the availability of clinical exams, muscle biopsies, and other testing, it is surmised that there may be a more important classification of myositis that physicians are not diagnosing which could possibly lead to improper treatment due to inaccurate diagnosis. There may be several types of immune and inflammatory myositis (IIM) that do not fit well into the typical sub classifications of myositis.

Retrospective and prospective data will be collected from physicians who have prescribed Acthar to myositis patients to determine what specific characteristics each patient has based on biopsy analysis, laboratory results, and clinical exams. Through biopsy analysis, subcategories of IIM will be determined and could illustrate which of these IIMs may be more responsive to Acthar therapy.

Inclusion Criteria:

1. Age 18-85
2. Male or Female
3. Clinical or pathologic diagnosis of polymyositis or dermatomyositis
4. Capable of providing informed consent and complying with treatment regimen

Exclusion Criteria:

1. History of scleroderma, osteoporosis, systemic fungal infections, ocular herpes simplex
2. Recent surgery, history of or the presence of a peptic ulcer, congestive heart failure, uncontrolled hypertension, or sensitivity to proteins of porcine origin
3. Any other co-morbid condition which would make completion of the trial unlikely
4. If female, pregnant or breast-feeding; or, if of childbearing age, an unwillingness to use appropriate birth control

All sites chosen are well-established universities, academic centers or private practices that specialize in neuromuscular diseases. These are physicians who have several myositis patients and who are prescribing or would prescribe Acthar to myositis patients