Myasthenia-like symptoms and Eaton-Lambert myasthenic syndrome have been reported in patients with polymyositis. On the other hand, the muscular involvement in myasthenic patients is well established. Thus difficulties in the differentiation of patients with polymyositis and myasthenic syndrome and myasthenia with myopathy may arise. The aim of this investigation was to re-establish the clues for distinguishing between different types of myasthenic syndromes. One hundred and twenty five patients took part in this investigation. According to the electromyography, biopsy and serum levels of creatine kinase data, the patients were subdivided into three groups. The first group consisted of 35 patients with data for chronic polymyositis. The second group consisted of 46 patients with myasthenia. The third group consisted of 44 patients with myasthenia and myopathy. Our data confirm the more often onset of myasthenia with ptosis and of myasthenic syndromes in polymyositis-from lower limbs. The bulbar involvement is more rare, while the autonomic nervous system involvement and decreased tendon reflexes are more common in patients with Eaton-Lambert myasthenic syndrome as compared to patients with myasthenia. Bulbar involvement is more typical for myasthenia, while four limbs involvement is more common in patients with myasthenia and myopathic changes. There are no differences in the typical decrementing response between patients with polymyositis, myasthenia or myasthenia with myopathic involvement. The Eaton-Lambert myasthenic syndrome is quite different and is found only in a small part of polymyositis patients. In conclusion the differential diagnosis between myasthenic syndromes in polymyositis and myasthenia with muscular involvement is not possible only by electromyography.