Lymphoid interstitial pneumonia (LIP) is presently regarded as a collection of dense lymphoid tissue within the lung that is highly suggestive of, and must be differentiated from, a lymphoproliferative disorder, specifically small B-cell lymphomas of extranodal marginal zone type, so-called MALT (lymphomas of mucosa-associated lymphoid tissue). LIP is associated with immunodeficiency. It is found in adults and is common in children. Idiopathic LIP may overlap histologically with the cellular form of nonspecific interstitial pneumonia (cellular NSIP). The histologic changes of LIP may be found in a variety of systemic diseases, including infections such as pneumocystis, Epstein-Barr, and HIV; autoimmune diseases such as Hashimoto thyroiditis and pernicious anemia; and connective-tissue diseases such as rheumatoid arthritis and systemic lupus erythematosus. LIP pattern may also be identified in patients with drug-related or toxin-related lung injury. LIP typically presents with symptoms of the underlying systemic condition. Prognosis is usually related to the severity of the patient’s underlying systemic disease. Corticosteroids may be of benefit in symptomatic patients.