Thymomas are unique tumours: a) because of their immense histogenetic heterogeneity b) on account of the varied clinical manifestation of associated paraneoplastic autoimmune diseases with a high incidence in as many as 35 % patients. The most frequent autoimmune disease is in 30% patients myasthenia gravis. The antibody formation in patients with myasthenia with a thymoma differs from the pathogenesis of nonthymomatous myasthenia gravis. Antibodies against acetylcholine receptors in patients with a thymoma are produced in lymph nodes, the spleen and bone marrow where T cells derived from thymomas circulate. In the tumourous epithelium of thymomas only epitopes are found such as titin, ryanodine receptor, neurofilaments which only imitate epitopes of the main autoantigens, i.e. acetylcholine receptors. Other autoimmune paraneoplastic diseases are found only in 5% patients with thymomas. Most frequently autoimmune diseases of the thyroid gland are involved, other endocrinopathies, haematological diseases, rheumatoid and skin diseases. In 2–3% patients with thymomas there is an association of myasthenia gravis with another autoimmune paraneoplastic disease. This pertains to several neurological diseases with known autoantigens and confirmed autoantibodies or autoreactive T cells – neuromyotonia, peripheral sensomotor neuropathy, stiff person’s syndrome, rippling muscle disease limbic encephalitis. Morvan’s syndrome pseudoobstruction of the gut. As to autoimmune diseases with unknown autoantigens polymyositis and dermatomyositis occur. In one third of the patients thymomas are manifested by local symptoms – chest pain, cough, dyspnoea, hoarseness. The main methods in the diagnosis of thymomas are CT and MRI. Accurate assessment of the type of thymoma according to the new WHO classification is possible only after histopathological and immunohistochemical examination. The method of choice in the treatment of thymomas is radical transsternal thymectomy. Before the era of immunosuppressive treatment the lethality of patients with myasthenia and thymoma after surgery was as high as 50%. Therefore in cca 85% patients with myasthenia first immunosuppressive treatment is recommended. Thymectomy is indicated only after optimal clinical improvement is achieved (pharmacological remission or marked improvement of the condition). When this procedure was applied during the last 18 years in a group of 98 patients with thymoma and myasthenia the lethality declined to 2%. In indicated cases, but always after surgery, radiotherapy is very effective, thymomas being very radiosensitive. Chemotherapy is prescribed only rarely in intrathoracic and remote metastases. In clinical manifestations of thymoma paraneoplastic autoimmunity, in particular in neurological diseases, immunosuppressive treatment is always indicated.

        Key words: thymomas, impaired immunotolerance, mechanisms of paraneoplastic autoimmunity, associated autoimmune neurological diseases, radical thymectomy, immunosuppressive treatment