It is well known that the biological behavior of lung cancer varies according to 
histological type and growth pattern. Therefore, more precise analysis of 
various morphologic features affecting prognosis are needed based on tumor 
histology. Lung cancer is mainly classified into 4 major histological types; 
squamous cell carcinoma, small cell carcinoma, adenocarcinoma and large cell 
carcinoma. Recently lung cancer has been subclassified into small cell carcinoma 
and non-small cell carcinoma on the basis of responsiveness to chemotherapy and 
radiation therapy. In small cell carcinoma, combination of chemotherapy and 
radiation therapy is the main modality for treatment, and the most important 
prognostic factor is LD and ED stage classification. Even so, a new histological 
classification is proposed according to the responsiveness to chemotherapy and 
radiation therapy. On the other hand, surgical therapy is the first choice for 
the treatment of non-small cell lung cancer and the most important prognostic 
factor is TNM and related stage classification. In squamous cell carcinoma, 
moreover, extended resection is sometimes tried because of its local 
invasiveness. Early lung cancer of hilar type is mostly squamous cell carcinoma 
and highly curative by resection. The biological behavior of adenocarcinoma is 
the most variable among lung cancers. In the histopathological study of 
surgically resected cases, moreover, histological differentiation, nuclear 
atypia of tumor cells, mitotic frequency, grades of scarring associated with a 
tumor, and degrees of infiltration of T-zone histiocytes is closely related to 
prognosis. Scoring of these factors may help a clinician to reach a decision on 
the necessity and type of postoperative adjuvant chemotherapy, particularly in 
cases of Stage I adenocarcinoma. Most cases of large cell carcinoma including 
giant cell carcinoma ultrastructurally reveal features of differentiation toward 
adenocarcinoma and/or squamous cell carcinoma. Giant cell carcinoma shows the 
most unfavorable prognosis because of its rapid growth. However, among operable 
cases of giant cell carcinoma, some long-term survivors do exist. Evaluating 
these forms of biological behavior according to tumor histology at the time of 
treatment, it is easier to decide whether or not adjuvant therapy is necessary.