A 50-year-old male was admitted to our hospital because of an abnormal shadow in chest X-ray films. The diagnosis of adenocarcinoma was obtained by transbronchial brushing cytology. Right lower lobectomy with hilar and mediastinal lymph node dissection was performed. Histological examinations of the resected specimen showed that the tumor was composed predominantly of spindle-shaped cells containing scattered islands of moderately differentiated adenocarcinoma. Immunohistochemically, the spindle cells were positively stained by vimentin, keratin and epithelial membrane antigen, suggesting sarcomatous transformation of adenocarcinoma cells.