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Adult T-cell leukemia (ATL) is a tumor of CD4-positive T cells that accompanies an infection by human T-cell lymphotropic virus (HTLV-I). ATL is classified into four types—acute, lymphomatous, chronic, and smoldering. Opportunistic infections are known to occur in patients with acute or lymphomatous type ATL; however, whether patients with chronic or smoldering ATL also have a high risk of opportunistic infections is not yet known. Herein, we report a case of pneumocystis pneumonia in a patient with smoldering ATL. He was a 64-year-old man with primary complaints of cough and dyspnea on exertion. A chest radiograph showed infiltration shadows in the left lung field. He was prescribed antibiotics for pneumonia; however, his symptoms worsened, and he developed hypoxemia. White-blood cell count was 13000/μL, and 7% of atypical lymphocytes were found in the smears of peripheral blood cells. His serum β-D glucan concentration was increased to 85.9 pg/mL, and his serum tested positive for anti–HTLV-1 antibody. Chest-computed tomography revealed diffuse ground-glass opacities in the bilateral lung fields. Pneumocystis-polymerase chain reaction performed on bronchoalveolar lavage fluid confirmed pneumocystis, but atypical lymphocytes were not detected via transbronchial lung biopsy. Therefore, he was diagnosed with pneumocystis pneumonia associated with smoldering ATL. Sulfamethoxazole-trimethoprim and corticosteroid therapies were administered to treat the pneumocystis pneumonia, and his symptoms and lung shadows improved rapidly. Thus, opportunistic infections, including pneumocystis pneumonia, may be caused by smoldering ATL. In the case of atypical lymphocyte detection in peripheral-blood smears, clinicians should consider the possibility of ATL.