First described in 1966 by Carrington and Liebow, lymphocytic interstitial pneumonitis (LIP) is a rare form of idiopathic interstitial lung disease. It is characterized by infiltration of the interstitial and alveolar spaces by lymphocytes, plasma cells, and other lymphoreticular cell types. The etiology of LIP is not completely elucidated, although both autoimmune and infectious possibilities have been postulated. LIP has been observed to be associated with numerous autoimmune processes, such as pernicious anemia, rheumatoid arthritis, systemic lupus erythematosus, Sjögren syndrome, and chronic active hepatitis. A possible infectious etiology is also suggested by the increased incidence in patients with HIV, especially in children. Another virus that has been associated with LIP is Epstein-Barr virus (EBV). Biopsy specimens of patients with LIP showed a significant increase in EBV genome compared to control subjects with other types of interstitial lung disease. Human T-cell lymphotropic virus type 1 has also been implicated.