Out of 700 patients operated on account of myasthenia gravis, 144 cases with tumours (20.57%) were evaluated. Classification took place in five groups: 1. Thymoma and myasthenia gravis; 2. Thymic cysts and myasthenia gravis; 3. Thymoma and latent myasthenia gravis; 4. Paraneoplastic myasthenia-syndrome; 5. Thymoma, myasthenia gravis and myasthenia-syndrome. Up-to-date classification of thymomas is: epithelioma with minimal, marked or overwhelming lymphatic reaction. Dark-cell and light-cell epitheliomas equally associate with myasthenia. Concerning prognosis, location of the tumour to thymic capsula, surrounding, perithymic tissue is more important than the histologic structure of the tumour. Previous examination in the case of mediastinal tumour can reveal latent, mild myasthenia which was not known till that time. Term of postthymectomic myasthenia can be excluded on the basis of these examinations. Heterogeneity is considerable among myasthenia-syndromes concerning both histopathologic and clinical features. There is a double indication of operation in the case of myasthenia with tumour: the tumour and the disease. Good results can be achieved in myasthenia gravis associated with tumour by means of total operation, post-operative radiation and by other up-to-date therapeutical procedures. The results are hardly beyond those of non-tumours myasthenia gravis.