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 Sj$check{o}$grens 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 autoimmune disease or HIV infection. He was admitted as a result of severe hypoxemia showing $PaO_2$ of 48.3mmHg. The patient was treated with corticosteroids after the diagnosis and had fully recovered without a sequalae or relapse.