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In view of the fact that certain objective determinants are needed to make a more reliable classification of nonsmall cell lung carcinomas (NSCLC) and aiming to assess the relationship between immunohistochemical typing and histopathological typing, immunohistochemical typing was done in patients with stage IV NSCLC using monoclonal anti-cytokeratin 18 (CK18) antibody. Also, to examine the role of immunohistochemical typing in predicting the prognosis of NSCLC, the relationships between the immunohistological type and response to chemotherapy, as well as length of survival, were evaluated. Tissue samples of 27 patients with stage IV NSCLC were examined immunohistochemically with anti-cytokeratin 18 (CK18) monoclonal antibody using “biotin-streptavidin” method. There was no relationship between immunohistochemical typing and histopathological typing, but the relationship between CK18 positivity and adenocarcinoma was statistically significant. The relationship between immunohistochemical typing and survival time, and that of histopathological typing and response to chemotherapy were not statistically significant, while a statistically significant relationship was found between immunohistochemical typing and response to chemotherapy. Response to chemotherapy was better in patients with CK18 (-) than in those with CK18 (+)■ We conclude that determination of CK18 pattern can be a useful criterion in deciding on the initiation of chemotherapy.