Idiopathic inflammatory myopathy (IIM) refers to the immune mediated myopathies associated with inflammatory cellular infiltrates in skeletal muscle tissue, and polymyositis (PM) and dermatomyositis(DM) are the two most important IIM with distinct clinical and pathological features.With regard to the clinical features, DM is distinct from PM in many aspects.The most distinguishable feature in DM is its association of typical skin lesions such as heliotrope rash, Gottron"s papule, and periungal erythema.While DM is considered as a paraneoplastic disorder, the relationship with a cancer is not clear in PM.While DM frequently affects children, PM almost always affects adults aged more than 18.