Forty-six thymomas, defined as neoplasms of thymic epithelium, were assessed in respect to histologic type, clinical manifestations and end results. Twenty-three were lymphoepithelial, 12 epithelial, and 11 spindled cell tumors. The sex distribution for all thymomas was approximately equal for males and females but the lymphoepithelial tumors were more common in females and the epithelial tumors more common in males. There was no distribution difference in sex for the spindled cell thymomas which occurred much later in life than the other types. Twenty-five percent of patients were asymptomatic, the tumors being discovered on roentgenograms done on a routine basis or for an unrelated purpose. The most common presenting symptoms were related to myasthenia gravis, and symptoms due to pressure on mediastinal structures were next in frequency. Other endocrine abnormalities were present in 20% of patients. Two male patients also had carcinomas of the thyroid gland. Twenty thymomas were infiltrative tumors having invaded the adjacent mediastinum. There were no examples of extrathoracic spread. Twelve of 16 patients with myasthenia gravis had lymphoepithelial thymomas and four epithelial thymomas. There were no cases of myasthenia gravis in patients with spindled cell thymomas. The crude five-year survival rate was 65%. Only 17% of patients died as a result of recurrent and infiltrative thymomas; the other 18% of patients died from a variety of causes not directly related to the thymomas. The epithelial type of thymoma tended to be more extensive and to pursue a more aggressive course but histologic classification was of little value in predicting final outcome. Poor prognosis is more closely associated with tumors having an infiltrative character. Classification is of importance, however, in alerting the clinician to certain associated diseases.