To search for the signaling events in lung carcinoma relevant to its tumorigenesis, we investigated the phosphorylation of MAPK and STAT3 in human lung carcinoma tissues and their paired normal tissues. Although relative amounts of MAPK levels differed among the cases examined, no clear difference in MAPK levels was observed between tumor tissues and their paired normal tissues. Of 79 cases examined, only 7 cases (8.9%) showed tumor-specific phosphorylation of MAPK, whereas 41 cases (52%) showed more than 2-fold lower levels of MAPK phosphorylation in tumor tissues. In contrast to MAPK, 42 cases (53%) showed tumor-specific increase in STAT3 expression. In addition, 15 cases (19%) showed tumor-specific increase of STAT3 phosphorylation and 51 cases (65%) had STAT3 phosphorylation proportional to its expression. Moreover, exogenous expression of either JAB or dominant negative STAT3 in lung carcinoma cell lines led to the suppression of STAT3 phosphorylation and the drastic reduction of anchorage-independent growth of the cells. Taken together, our results suggest that JAK-STAT3 signaling has a pivotal role for oncogenic growth of lung carcinoma cells.