A 46-year-old man whose parents were from Nagasaki had a 24-hour bath system in his house. He had had a cough for 26 years and dyspnea for 2 years. He consulted our hospital because of the increase of his cough and dyspnea. Chest X ray and CT showed abnormal shadows in both lung fields and Legionella pneumophila type 3 was detected by sputum cultivation. The condition which appeared this time responded to antibiotic medication. However, the abnormal shadows in both lung fields and the abnormalities in respiratory function remained. The remaining abnormal shadows in both lung fields were considered to be the cause of his dyspnea for 2 years and cough for 26 years. Atypical lymphocytes with a floriform nucleus were observed in peripheral blood. Gene analysis detected monoclonal human T lymphotropic virus type I (HTLV-I) provirus DNA. We diagnosed as smoldering type adult-T-cell-leukemia (ATL). Thoracoscopic lung biopsy revealed fibrotic thickening of the interstitial tissue accompanied by structural destruction. The pathological changes in both lung fields were diagnosed as HTLV-I related lung disease and infiltration of ATL. Known pathogens of lung infection accompanying ATL include viruses, acid fast organisms, and fungi. Legionella pneumonia happened to be the opportunity leading to the diagnosis of HTLV-I related lung disease is this case.