This study was aimed to evaluate the prevalence and prognostic implication of thyroid transcription factor-1 (TTF-1) immunoreactivity in 81 human lung carcinomas, including 65 cases of non-small cell lung carcinoma (NSCLC) and 16 cases of small cell lung carcinoma (SCLC); and also to investigate its relationship with the cell proliferation and regulation by immunostaining of Ki-67 and p53 proteins, respectively. The immunohistochemical staining for TTF-1 (clone 8G7G3/1) was performed and several clinicopathologic variables and the follow-up data were obtained. The immunostaining results for TTF-1 were semiquantitatively interpreted as negative and positive. Of NSCLCs, TTF-1 is highly expressed in adenocarcinomas (76%), whereas squamous cell carcinomas revealed no immunoreactivity (0%). SCLCs showed strong TTF-1 expression (88%). In NSCLC, TTF-1 expression was inversely correlated with Ki-67 proliferative activity and independent of p53 overexpression. TTF-1 (+) group tended to show better survival than TTF-1 (-) group in NSCLC. Conclusively, these observations suggest that TTF-1 is a sensitive and specific diagnostic marker for pulmonary adenocarcinomas and SCLCs; that TTF-1 might have a good prognostic implication based on its inverse correlation with Ki-67 proliferative activity and tendency for better survival in NSCLC; that this cell lineage marker may play a role in the molecular pathogenesis of lung cancers at the level of transcription.