A 65-year-old man was found to have a mass lesion surrounded by ground-glass attenuation in the left upper lobe on chest radiography. He was diagnosed with stage IA adenocarcinoma of the lung. The resected lung specimen revealed papillary adenocarcinoma associated with infiltralion of numerous lymphocytes in the alveolar septa, which was consistent with focal lymphocytic interstitial pneumonia (LIP). However, it was not associated with Sjögren's syndrome or any other immunologic abnormalities. Immunohistochemical study disclosed that CD8 positive T-cells constituted the major element of the infiltrated lymphocytes in the tumor, and were also found in the enlarged alveolar septa, suggesting an association between lung cancer and LIP. To our knowledge, this is the first description of an association between LIP and lung cancer. In addition, the focal LIP in this case probably reflected local immune response to an antigenic stimulus caused by lung cancer.