A collective of 38 patients with a thymoma, confirmed by microscopic investigation, was analysed retrospectively. 63% of them were female. In 27% of the cases the final diagnosis could be made by autopsy only. 56% of the operated patients lived half of their age-dependant expectation of life. The 5- and 10-year-survival rates amount 84% and 68%, respectively. Of the non-operated patients no one was alive 2.5 years after diagnosis. There was no consistent relationship between the different histologic structure of thymomas (degree of lymphocytic infiltration) and the clinical behaviour and the survival rate. All patients with "parathymic syndromes" were women. One case with pure red cell anemia (erythrophthisis) was associated with spindle-shaped epithelial thymoma. The incidence of myasthenia gravis did not depend on the histologic pattern of thymomas.