BACKGROUND Myasthenia gravis (MG) is a disease with autoimmune background. Impaired neuromuscular transmission is caused by blockage of acetylcholine receptors on postsynaptic membrane by circulating specific antibodies. Recognition of myasthenia gravis in children, especially its ocular type, may be difficult due to occurrence of similar clinical symptoms in other diseases like ecephalomyopathies. MG is characterized by variety of clinical symptoms and their alternations during excercise and rest. OBJECTIVE Case report of nearly 18-year-old girl with generalized type of MG. METHODS Before hospitalization the girl had been treated psychiatrically for 6 months due to suspicion of conversion disorders. After performance of clinical test and electro-neurophysiological examinations mysathenia gravis was diagnosed and conservative treatment was instituted. Additionally, on the ground of low effectiveness of the treatment, sterydotherapy and immunosuppressive treatment were instituted without marked clinical improvement. Thymectomy was also low effective. Thymic inflammation was recognised histopathologically. Only after performance of 5 plasmapheresis was significant clinical improvement achieved. CONCLUSIONS Plasmapheresis may be used not only in the treatment of myasthenic crisis but also in the treatment of drug-resistant mysthenia gravis.