This is a phase 3, 56-week, multicenter, randomized, placebo controlled, double-blind study of IgPro20 (subcutaneous Ig) treatment in adult subjects with dermatomyositis (DM). Subjects will be randomized to 1 of 2 treatment sequences: Sequence A: 0.5 g/kg IgPro20 for 24 weeks followed by 0.5 g/kg IgPro20 for 28 weeks or Sequence B: placebo for 24 weeks followed by 0.5 g/kg IgPro20 for 28 weeks. Screening: about 2 months, 52-week treatment period. Duration of the study will be approximately 3.5 years. A sufficient number of subjects will be screened in order to randomize 126 subjects into the study. Key eligibility criteria: Adults with moderate to severe DM with failure of other therapies. Main study interventions: vital signs, weight, labs (safety + specialty), selective PKs, ECGs, home infusions of self-administered study drug (weekly for 52 weeks), patient diaries, clinical assessments, patient questionnaires, optional future research sub-study including genetic testing. Some of the patient and physician assessments will be administered by tablets. Subjects will receive materials (Crono pump) and instructions from the research staff for home-based self-administration of the study drug via subcutaneous infusions.

You may be eligible for this study if you meet the following criteria:

• Conditions: Dermatomyositis
• Age: 18 years - 99 years
• Gender: All