AbstractInterstitial pneumonia unrelated to Pneumocystis carinii or other infections was observed histopathologically in 5 of 25 rhesus monkeys infected with simian immunodeficiency virus (SIV). The predominant lesion was lymphocytic infiltration of interalveolar septa and hyperplasia of peribronchial and perivascular lymphoid tissue. Immunohistochemical staining using a panel of antibodies against human T and B lymphocytes, macrophages, and immunoglobulins showed that peribronchial aggregates and interstitial infiltrates were predominantly B cells, whereas perivascular masses consisted mainly of T cells. One animal with a primary B‐cell lymphoma of the spinal cord had secondary plasmacytoid lymphomatous nodules throughout the lung which were accompanied locally by reactive B‐cell lymphoid follicles. Another animal also had large areas of diffuse alveolar fibrosis and epithelial metaplasia to a bronchiolar type. In two monkeys, branches of the pulmonary arteries showed intimal proliferation and organizing occlusive thrombi, some of Which were mineralized.