Lymphocytic interstitial pneumonia(LIP) is characterized by a massive infiltration of the interstitium of the lung by mature lymphocytes, plasma cells and reticuloendothelial cells. LIP may be associated with autoimmune diseases including Sjogrens syndrome, SLE, myasthenia gravis, pernicious anemia, autoimmune hemolytic anemia, and HIV or an EB virus infection. There is a possibility of LIP progressing to a pulmonary or systemic lymphoma. The therapeutic response to corticosteroids and/or immunosuppressive drugs varies. Here we report a case of LIP that was diagnosed by an open lung biopsy and clonality study. The patient was a 36 year-old man without antuimmune disease or HIV infection. He was admitted as a result of severe hypoxemia showing PaO₂ of 48.3 mmHg. The patient was treated with corticosteroids after the diagnosis and had fully recovered without a sequalae or relapse.