THREE GROUPS OF PRIMARY INFLAMMATORY MUSCLE DISEASES: The primary inflammatory muscle diseases comprise three main subsets: polymyositis (PM), dermatomyositis (DM) and inclusion body myositis (IBM). PM and DM are characterized by a proximal weakness that develops along weeks to months and by elevated creatine phosphokinase levels. Cutaneous involvement including both erythema and edema and infantile or adult onset are DM specific. PM and IBM only concern adults. Several PM/DM manifestations must be searched for because of their severity: swallowing disorders, various mechanisms of respiratory dysfunction (swallowing pneumopathies, interstitial lung disease, respiratory muscle deficiency) and cardiac involvement. DIAGNOSTIC ELEMENTS FOR PM AND DM: Two investigations, beside biopsy, are particularly useful: muscle MRI imaging showing inflammatory pattern and specific detection of antisynthetase autoantibodies (PM/DM with interstitial lung disease) and anti-Mi-1 and 2 in DM. PHYSIOPATHOLOGICAL DATA: PM and DM differ in their histological and physiopathological characteristics: perivascular B and CD4 lymphocyte infiltrates and complement deposits at the origin of humoral induced vascular disease in DM and perimysial CD8 lymphocytes inducing a cellular mediated cytotoxic injury in PM. Class I HLA antigen expression on the muscle fibers and production of cytokines play a crucial role in the pathogenesis of these two diseases. PM and DM may be associated with cancers, connective-tissue disease (overlap syndrome). Some PM are secondary to HIV, HTLV1 virus and toxoplasmosis infection. CHARACTERISTICS OF INCLUSION BODY MYOSITIS: IBM, the most frequent acquired myopathy after 50 years of age, is characterized by particular features: not only clinical (late onset, selective weakness, early distal involvement, slow course, unresponsiveness to corticosteroid and immunosuppressant agents); but also histological (rimmed vacuoles, filamentous inclusions) and pathogenic (cytotoxic and degenerative inflammatory process, similar to Alzheimer's disease, with beta-amyloid protein accumulation).