Fifty-six cases of surgically resected adenosquamous carcinoma of the lung were studied clinicopathologically, and their outcome was compared with that of adenocarcinomas and squamous cell carcinomas of the lung. The frequency rate of adenosquamous carcinoma was 2.6% of 2160 primary lung cancers resected in the National Cancer Center Hospital (Tokyo, Japan). The survival curves of patients with adenosquamous carcinomas, adenocarcinomas, and squamous cell carcinomas indicated that the outcome of adenosquamous carcinoma was poorer than that of adenocarcinomas and squamous cell carcinomas, particularly in Stages I and II. The amount of adenocarcinoma component did not affect the survival rate, although the histologic features of metastatic lymph nodes was somewhat influenced by the histologic type of the primary tumors. The histologic subtype of adenosquamous carcinoma was one of the independent prognostic determinants.