Chronic active Epstein-Barr virus (CAEBV) infection is characterized by persistent infectious mononucleosis-like symptoms, an unusual pattern of Epstein-Barr virus (EBV) antibodies, detection of the EBV genome in affected tissues or peripheral blood, and chronic illness that cannot be attributed to any other known disease. This is the first reported Korean case of an immunocompetent adult with CAEBV-associated interstitial pneumonitis. A 28-year-old female was admitted with a fever that persisted for 3 weeks. She had multiple lymphadenopathy, hepatosplenomegaly, pancytopenia, and elevated serum aminotransferase levels. Serology for antibodies was positive and chest computed tomography showed diffuse ground glass opacities in both lungs. Histopathology of the lung tissue showed lymphocyte infiltration, and EBV DNA was detected in those lymphocytes using in situ hybridization with an EBV-encoded RNA probe. After 1 month of hospitalization, she improved without specific treatment.