In 54 patients with thymoma, the factors having the greatest bearing on prognosis were presence or absence of gross tumor invasion or an associated clinical syndrome of myasthenia gravis, anemia due to erythrocytic hypoplasia or hypogammaglobulinemia. Of 17 patients with invasive tumors, 14 were either dead of tumor or living with disease, while 34 of 37 patients with encapsulated tumors were either living or dead of other causes without evidence of neoplasm. A clinical syndrome associated with thymoma was present in 15 patients. Thymectomy appeared to have little if any effect on the syndrome; in three instances the syndrome appeared after removal of the thymoma. Analysis of the histological findings, including type of epithelium, degree of lymphocytic infiltration and presence of other patterns such as rosettes, perivascular spaces and germinal centers, showed that there was no consistent relationship between the microscopic appearance of thymomas and their clinical behavior. Knowledge of the striking variations in histopathologic features is necessary in differential diagnosis.