Specimens of Mediastinal lesions and lung masses collected by fine needle aspiration (FNA) was examined with both light and transmission electron microscopy. Sixty eight specimens (9 mediastinal and 40 lung masses, 4 metastatic chest masses and 15 metastatic lymph nodes from lung cancer) from 104 cases (60.5%) were satisfactory for evaluation. With light microscopy, the nature of the lesions was assessed and the ultrastructural features of the lesions were evaluated with transmission electron microscopy. Diagnosis of histologic type of the lesions by light microscopy was consistent with that by transmission electron microscopy at a conformity rate of 50.4%. The results indicate that ultrastructural features are valuable in ascertaining histologic type of the lesions.