The fine needle aspiration (FNA) cytologic findings in 16 cases of histologically confirmed thymoma are reported. The aspirates were obtained under fluoroscopic guidance. The cytologic diagnoses were inadequate sample in one case, thymoma in 12 (75%), small cell carcinoma or thymoma in 1, benign mesenchymal tumor in 1, and germ cell tumor in one. The cytologic features were detailed according to the constituent epithelial cell type, and to the ratio of epithelial cells and lymphocytes. Fifteen cases were classified into 4 small epithelial cell type, 6 intermediate epithelial cell type, 1 large epithelial cell type, 1 large pleomorphic epithelial cell type, and 3 spindle-shaped epithelial cell type. Cytologic differential diagnosis was discussed, and the important criteria for the cytologic diagnosis of thymoma were reviewed. This review leads us to think that nonoperative cytologic approaches in the diagnosis of the thymoma are possible, and that correct cytologic diagnosis of thymoma with FNAs can easily be made, if adequate samples are obtained. However, the invasiveness and histologic type could not be predicted by cytological features only. Knowing various cytologic and histologic features of thymoma will be helpful for the diagnosis of thymoma and the differential diagnosis of mediastinal tumors.