Thyroid transcription factor-1 (TTF-1), and cytokeratin 7 (CK7) and cytokeratin 20 (CK20) have recently been reported to be useful to distinguish between primary and metastatic lung adenocarcinoma. The aim of this study was to determine the usefulness of the staining patterns of pulmonary adenocarcinoma with antibodies to TTF-1, CK7, and CK20 in differentiating primary from metastatic pulmonary adenocarcinoma. Of the 66 lung adenocarcinoma specimens that were enrolled in our study, there were 40 primary lung adenocarcinomas, 12 metastatic adenocarcinomas from breast, 13 metastatic adenocarcinomas from colon, and 1 metastatic adenocarcinoma from stomach. The expression of TTF-1, CK7, and CK20 was assessed by immunohistochemistry. We found that 73% of primary lung adenocarcinomas expressed TTF-1, whereas all nonpulmonary adenocarcinomas lacked TTF-1 staining. CK7 expression was significantly more frequent in adenocarcinomas of pulmonary and breast origin than gastrointestinal (GI) origin (p < 0.001). In contrast, CK20 expression was significantly more prevalent in adenocarcinoma that originated in the GI tract than that of pulmonary or breast origin (p < 0.001). A combination of TTF-1+CK7+CK20- was highly significantly associated with primary adenocarcinoma of lung (vs GI tract, p < 0.001; vs breast, p < 0.001). A combination of TTF-1-CKCK20+ was highly significantly associated with adenocarcinoma of GI origin (vs lung, p < 0.001; vs breast, p < 0.001). Our study has confirmed that expression of CK7, CK20, and TTF-1 is a useful immunohistochemical marker for diagnosis of lung tumors and for differential diagnosis of primary pulmonary adenocarcinomas from extrapulmonary adenocarcinomas metastatic to the lung. Application of this panel of antibodies might be expected to increase the accuracy of diagnosis.