A clinicopathological study of surgically resected thymomas was performed using Masaoka's staging and modified Masaoka's staging systems, and the utility of these two staging systems was compared. The modification enabled adjustment for the disproportion in the number of cases between Stage I and Stage II. Analysis of survival rates, according to the tumor stage, indicated that the old classification should be reappraised, that is, division into non-invasive and invasive thymomas, although staging may contribute to the indication for postoperative radiotherapy, especially for Stage II disease. Analysis of the cases showed a wide spectrum of aggressiveness, varying from cases showing slow progression with a relatively favorable prognosis, such as the spindle cell type, to cases with rapid progression leading to tumor death in a relatively short time, such as the epithelial cell predominant and polygonal cell type. The pathological stage at the time of first surgical resection would reflect the degree of aggressiveness of thymoma in many instances. Therefore, not only staging the tumor extent but also grading of its aggressiveness are needed in order to predict the prognosis of patients with thymoma. For the latter, histology and cytopathology are helpful.