Diagnosis of Polymyositis or Dermatomyositis.

Baseline muscle weakness score of less than or equal to 139 out of 160 on manual testing (MMT).

Baseline functional assessment score of less than or equal to 82 out of 91.

Ability to provide informed consent to all aspects of the study after full information is provided.

Age equal to or older than 18.

A diagnosis of classic or definite polymyositis or dermatomyositis (Critieria A and B plus at least one of the three other criteria):

1. Symmetrical proximal muscle weakness;
2. Muscle biopsy abnormalities at some time during their disease:

i. degeneration and regeneration of muscle fibers

ii. necrosis

iii. phagocytosis

iv. interstitial mononuclear infiltration;

c. Elevation of serum creatinine phosphokinase (CPK), transaminases, lactic dehydrogenase (LDH) or aldolase activity;

d. Electromyography (EMG) triad of changes

i. small amplitude, short duration polyphasic motor unit potentials

ii. fibrillations, positive sharp waves, increased insertional irritability

iii. spontaneous bizarre high frequency discharges;

e. Typical skin rash of DM.

Willingness to undergo 2 muscle biopsies.

Evidence of active disease as measured by weakness, and an elevated CK or an active MRI.

Must be tapered to a stable dose of steroid equal to or less than 0.50 mg/kg/day equivalent of prednisone for one month prior to the study.

If on additional immunosuppressive drugs, the drugs must be maintained at a stable dose for 1 month prior to the initiation of therapy and will be maintained throughout the trial.

Women of childbearing potential and men whose partners are of childbearing potential must practice an acceptable form of contraception. No pregnant females or nursing mothers.

No history of hepatitis or abnormal liver function tests.

No history of prior thyroid disease.

No active acute or chronic infections requiring antimicrobial therapy, or serious viral or fungal infections.

No preexisting or coexisting malignancy other than basal cell and localized squamous cell carcinoma of the skin.

No history of cerebrovascular accidents, seizure disorder, aseptic meningitis, transverse myelitis or central nervous system disease.

No history of documented coronary artery disease, cardiomyopathy, greater than first-degree heart block, or dysrhythmia requiring therapy.

No confounding medical illness that in the judgement of the investigators would pose added risk for study participants.

No anemia requiring maintenance blood transfusions; leukoplakia with WBC less than 3,000 micrograms or absolute neutrophil count less than 2,000 micrograms; and platelet count less than 100,000 micrograms on at least two different occasions.

No history of (or current) autoimmune hemolytic anemia.

No current anticoagulant therapy.