Forty-eight-year-old male was introduced to our department because of the abnormality in chest XP. He did not have remarkable past history. He was never-smoker and had no history of exposure to the industrial materials. Physical examination showed no remarkable findings. Plain chest XP showed hazy nodular opacity in the right lung field with ill-defined margin (Fig. 1A). Chest CT showed that the tumor was disk like-shape in the posterior segment of right upper lobe (Fig. 1B and C). The lateral branch of the posterior segment of upper lobe penetrated through the center of the tumor. The margin was not clear in some part of the tumor and spiculation was observed. Size of the tumor was 28 mm x 26 mm x 9mm. There was no lymph node swelling in the hilum and mediastinum. Blood tests including tumor marker for lung cancer were unremarkable. PET-CT showed strong positivity (SUVmax of 10) in the tumor (Fig. 1D) without metastasis to the lymph nodes, pleura and the other organs. Magnetic Resonant Imaging (MRI) with contrast media for the brain showed no abnormality. Bacteriological studies for lung tuberculosis, non-tuberculous mycobacteriosis, fungus diseases were all negative. Histological examination using Hematoxylin-Eosin (HE) staining for the specimens obtained by transbronchial lung biopsy showed dense spindle cells with irregular shape nucleus and some necrosis. The pathological diagnosis was malignant myoepithelial tumor. Preoperative staging using chest CT, PET-CT and brain MRI was stage IA (T1N0M0). Right upper lobectomy with lymph nodes dissection were performed. Fig. 2A shows the macroscopic view of the tumor. The tumor was consisted of a cluster of irregular small nodules along the airways and the part of the margin was indistinct. No invasion was found in the lymph nodes and the pleura. Microscopic examination showed that the tumor was filled with dense spindle cells with pleomorphism and division of chromosome (Fig. 2B). The pathological diagnosis was grade 2 leiomyosarcoma using FNCLCC grading system as follows. The tumor differentiation was certain (score 2), mitotic count was 2-3/10 hyperpower fields (score 1) and tumor necrosis was 20% (score 1), the total score, all together, was 4. Post-operative staging with PET-CT and pathological grading was Stage IIA. The clear transition from normal smooth muscle bundle (Fig. 3A, thin arrow) to a cluster of malignant spindle cells was observed (Fig. 3A, thick arrow). Immunohistochemical staining routinely used for diagnosis of histological types of carcinoma such as CytokeratinMNF116, Cytokeratin5/6, P40 for the squamous cell carcinoma, TTF1, Napsin A for the adenocarcinoma, CD 56, Synaptophysin, ChromograninA and S100 for the neuroendocrine carcinoma were all negative (Table 1). Staining using CD 34 and D2-40 for vascular and mesothelial malignancies were also negative. On the other hand, although Desmin and Calponin were negative, Vimentin, H-Caldesmon, alpha-SMA and HHF35 were strongly stained suggesting that the origin of the tumor was smooth muscle. MIB-1 labeling index was 67.7%. Fig. 3B shows positive staining of H-Caldesmon for sarcoma cells as well as normal bronchial smooth muscle. Structure of the vessels in the tumor was preserved and smooth muscle cells of the vessels were normally stained (Fig. 3B, arrow heads). The Elastica HE staining (not shown) showed that destruction of elastic fibers was more prominent in bronchial walls than vessels.