Based on the inexistence of references in Brazilian and International literature of the association polymyositis, myasthenia gravis and thymoma in a young male adult, the authors propose to describe one case presenting the associated syndrome. The disease began when the patient was 22 years old, being the climax of its clinical expression four years later. It is emphasyzed the difficulty on the differential diagnosis between polymyositis and myasthenia gravis, once these diseases frequently have their clinical manifestations well individualized. In this study we call attention to the existence of clinical aspects of myasthenia gravis in patients presenting polymyositis and, on the other hand, histopathological alterations typical of polymyositis in myasthenic muscles, what makes the clinical diagnosis uncertain many times.