The epithelial-mesenchymal transition (EMT) is also involved in cancer progression and metastasis. The expression of mesenchymal marker genes might be associated with a more aggressive form of the disease. CD146 is a marker of mesenchymal cells, a potentially metastasis-promoting cell adhesion molecule, and its expression has been described in various types of solid tumors. We, herein, evaluated the expression of CD146 in patients with adenocarcinoma of the lung by using immunohistochemistry to clarify its clinical significance. Tumor specimens were collected from 183 consecutive patients who underwent complete resection for lung adenocarcinoma from 2003 to 2007 in our Department. We analyzed the CD146 expression levels in the primary lung adenocarcinoma by immunohistochemistry. Positive expression of CD146 was identified in 16 (8.7%) patients. A significant association was only observed between the CD146 expression level and male patient gender (p=0.03); other factors were not associated with CD146 expression. The 5-year overall survival (OS) rate in patients with tumors that were negative and positive for CD146 expression was 84.4% and 50.0%, respectively (p<0.01). A positive CD146 expression was found to be associated with the OS based on a univariate survival analysis (p=0.013). CD146 expression was more frequently detected in males than in females. The positive expression of CD146 was associated with a poorer OS according to the survival analysis. CD146 may be a useful marker for predicting poor prognosis in patients with NSCLC following complete resection.