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OBJECTIVES An adenosquamous carcinoma (ASC) of the lung is a relatively rare tumor. In this multi-institutional cohort study, we tested the hypothesis that an ASC exhibits more aggressive clinical behavior as compared to adenocarcinoma (AC) and squamous cell carcinoma (SC). METHODS This retrospective cohort study used a prospective database produced by the Japan National Hospital Organization Study Group for Lung Cancer over a 7-year period (operations from 1997 to 2003, follow-up data until March 2010). During that period, 4668 cases underwent an operation for various types of primary malignant lung tumors. When a sample from a tumor comprised at least 20% each of SC and AC, the case was classified as ASC. Pathologic staging was done according to the seventh edition of the International Union against Cancer (UICC) Tumor Node Matastasis (TNM) classification of malignant tumors. RESULTS We identified 114 patients with ASC (2.4%), 2993 with AC (64.2%), and 1369 with SC (29.3%). Kaplan-Meier survival curves for all stage cases, p-stage IA, IB, and IIIA tumors indicated that ASC cases had the least favorable survival. The 5-year survival rates for all stage cases were 23.3% for ASC, 58.0% for AC (p < 0.0001), and 40.8% for SC (p < 0.0001). The 5-year survival rates for p-stage IA were 42.0% for ASC, 81.8% for AC (p = 0.0005), and 63.4% for SC not significant (NS), while those for p-stage IB were 19.3%, 65.3% (p = 0.0024), and 46.8% (NS), respectively, and those for p-stage IIIA were 17.8%, 24.8% (p = 0.0154), and 18.8% (NS), respectively. There was a tendency for greater survival differences between ASC and AC in earlier tumor stages. A step-wise multivariable model demonstrated that sex, age, performance status, histology, tumor size, p-stage, operative method, and neoadjuvant/adjuvant therapy were independent prognostic factors. CONCLUSION ASC of the lung is more aggressive than AC and SC. The decreased survival of patients with ASC as compared with either of those single histology tumors suggests the need for a clinical trial of adjuvant chemotherapy that includes early-stage patients. [ABSTRACT FROM AUTHOR]