Idiopathic inflammatory myopathies are a group of acquired, heterogeneous, systemic diseases commonly regarded as autoimmune disorders. Differential diagnosis includes muscular dystrophies, especially the dysferlin-deficiency myopathy. We report a case of a patient diagnosed with polymyositis and with associated autoimmune diseases that finally turned out to be a dysferlin deficiency (limb girdle muscular dystrophy type 2B). A possible link between dysferlin deficiency an autoimmunity is discussed.