we present an investigation of a case of polymyositis affecting two sisters of one same parenthood. their cases have been documented for almost two decades, being investigated by means of a diagnostic protocol which combined clinical findings as well as laboratorial, histopathological and image tests. in both cases, clinical manifestations started in childhood, without signs of involvement of the central and peripheral nervous system. both patients proved to respond to a therapeutics based on corticosteroids. the degree of relatedness between their parents corroborate the notion that genetic factors may contribute to the development of the disease.