CASE PRESENTATION: A 50-year-old Chinese male presented with fever, pancytopenia and splenomegaly 4 years ago. Splenectomy was performed, but lymphocytosis was revealed in follow-up. He developed recurrent non-productive cough with exertional dyspnea in recent one and a half years, which temporarily improved after antibiotic treatment. However, serial chest CT scans showed progressive bilateral diffused pulmonary infiltrates (fig 1). Bone marrow examination prior to admission revealed a prominent lymphocytosis with a proportion of 96%, predominated by mature lymphocytes. Flow cytometry showed monoclonal CD3+CD8+TCRVβ cells, among which TCRVβ20 cells accounted for 84.93%. T-cell LGL leukemia was confirmed by transmission electron microscopy. He was admitted to our hospital for an unrelieved deteriorating episode twenty days ago. The peripheral leukocyte count was 51.89×109/L, with 90.3% lymphocytes. Bronchoscopy showed no bronchial mucosal congestion or purulent secretion. The cytometry of bronchoalveolar lavage fluid showed 22% macrophages and 78% lymphocytes. Video-assisted thoracoscopic lung wedge resection was performed, and the pathology showed perivascular infiltration of lymphocyte-like cells with large and irregular nuclear, consistently in size (figure 2). Immunohistochemical staining revealed CD3 (major+), CD5 (major+), CD8 (major+), CD20 (minor+), CD23 (-), CD38 (focal+), CD56 (-), cyclinD1 (-), CD4 (minor+), CD7 (minor+), κ (+), λ (+), TIA (+), GramB (+), Ki-67 (scattered +). Finally, the pulmonary infiltration was confirmed as a local involvement of T-cell LGL leukemia. Fifteen milligram (mg) of methotrexate per week and 40mg methylprednisolone per day was administered. His symptoms, count of lymphocytes and lung infiltrates were improved after treatment for a month.