A 69-year old woman presented with a 2-week history of cough and dysponea. Computed tomography (CT) revealed a 35 x 32 x 39 mm oval mass in the left hilar region, extending into the left main bronchus (Figure 1A, B). We performed the bronchoscopy and found a white mass extending into the left main bronchus from the bronchial orifice of the left upper lobe (Figure 2A, B). All specimens obtained from transbronchial biopsy (TBB) were composed of coagulative necrotic tissue (Figure 3A). Two weeks later, CT was repeated, and the scan showed that the mass had increased to 50 x 42 x 52 mm. Atelectasis was observed in the lingular segment of the left lung. Bronchoscopic re-examination revealed an extension of the mass to the vicinity of the tracheal bifurcation. First, we performed TBCB 10 times to collect the tissues present away from the tumour centre. At this time, the tumour appeared white without blood oozing, suggesting that it should be all necrotic. Subsequently, we carried out additional TBCB and TBB for a total of 23 times to obtain the specimens of tissues located near the tumour centre. As the examination progressed, it became possible to confirm the slight reddish surface and blood oozing to some degree during the biopsy (Figure 2C), so we considered that we had taken sufficient viable tissues. The former specimens were all necrotic, but the latter showed spindle-shaped and round-shaped atypical cells with eosinophilic cytoplasm (Figure 3B). Immunohistochemical analysis was positive for alpha-smooth muscle actin (Figure 3C) and HHF35, but negative for AE1/AE3, TTF-1, p40, CK5/6, and CD34, leading to a diagnosis of leiomyosarcoma. Neither metastases nor other primary sites were found after positron emission tomography-CT with 18F-fluorodeoxyglucose. Thus, the patient was diagnosed with primary pulmonary leiomyosarcoma. Surgery with total lung resection was difficult due to vascular invasion. Since there is little evidence of application of chemotherapy and radiation therapy in primary pulmonary leiomyosarcoma, we administered proton therapy at a total dose of 66 Gy in 25 fractions over a period of 8 weeks to achieve a strong local therapeutic effect. The tumour gradually shrank to 43 x 27 x 37 mm for 8 months post-proton therapy, along with atelectasis resolution (Figure 1D).