Pulmonary adenosquamous carcinoma is a rare malignant tumor as defined by the Japan Lung Cancer Society Classification. At our institution, of the 1,023 patients who underwent resection for primary lung cancer, 42 (4.0%) had adenosquamous carcinoma. Here, we present the clinical features of this malignant tumor. The male : female ratio was low. Many tumors were located peripherally, and the positive rate for carcinoembryonic antigen (CEA) was 54.8%; these clinical findings were similar to those of adenocarcinoma. On the other hand, many tumors had relatively large diameter, and most of the patients were heavy smokers; these findings were consistent with those of squamous cell carcinoma. Hence, the cases of adenosquamous carcinoma had the characteristics of both adenocarcinoma and squamous cell carcinoma. The prognosis of patients with adenosquamous carcinoma was poorer than those of patients with adenocarcinoma and those with squamous cell carcinoma, irrespective of whether it was stages I or II. Adenosquamous carcinoma is characterized by a highly aggressive biological behavior and a high rate of early metastasis. Therefore, even if the diagnosis is made at an early phase, an aggressive approach, including adjuvant chemotherapy, might be necessary for adenosquamous carcinoma.