Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) are acquired inflammatory muscle disorders of unknown etiology that are currently classified under the rubric of idiopathic inflammatory myopathy (IIM). All forms of IIM are characterized clinically by muscle weakness and pathologically by chronic muscle inflammation, although systemic manifestations involving many organ systems are common. Clinical subgroups have been recognized for many years and include adult-onset and juvenile-onset forms of PM and DM, myositis associated with another connective tissue disorder or malignancy, and the more recently recognized entity termed inclusion body myositis (1,2). The presence of characteristic cutaneous features such as Gottron’s papules and sign or the classic heliotrope rash distinguish DM from PM, and skin lesions may dominate the clinical presentation in some instances. Although the IIMs are rare diseases with an estimated incidence of 10 cases per million per year (3), they are associated with substantial morbidity and mortality. Currently available treatment of IIM is unsatisfactory. Corticosteroids are the only agents approved by the US Food and Drug Administration (FDA) for myositis. However, anecdotal reports and reports of case series do support the use of other immunosuppressive agents such as methotrexate and azathioprine in the treatment of myositis. Because of the rarity and heterogeneity of IIM, there is a paucity of controlled prospective clinical trials, and most published studies are from single referral centers reporting retrospectively on small numbers of patients followed up for relatively brief periods of time. Furthermore, accurate assessment of the effects of therapeutic interventions in IIM has been hampered by unreliable and insensitive outcome measures, as well as the challenge of distinguishing active and reversible disease from irreversible damage to muscle and other organs. Although previously published trials have utilized predefined outcome measures, no uniform criteria have been used to guide the conduct of these trials. As a result, different inclusion and exclusion criteria, trial duration, frequency of assessments, concomitant therapies, safety monitoring procedures, outcome measures, definitions of responses and disease flares, and stratifications of patients at outcome analysis have been used in almost all trials. Thus, it has been difficult to compare results from any two myositis trials even when the same agent is being studied, resulting in the current uncertainty regarding the safety and efficacy of most agents.