When an adult suffers from muscular symptoms, the diagnosis of polymyositis is often accepted if muscular biopsy reveals necrosis, fibrosis and cellular infiltrate with high expression of major histocompatibility complex class I. Late-onset limb-girdle muscular dystrophy (LGMD) can also be considered. We report the case of a young woman who suffers from dysferlin deficiency, and who was mistakenly treated for refractory polymyositis for 5 years. In LGMD, standard pathological analysis can indeed wrongly give a diagnosis of polymyositis. Immunofixation must be performed to avoid this mistake.