Lymphocytic interstitial pneumonia is a common complication of HIV infection in children, but uncommon in adults. It is characterized clinically by the presence of cough and dyspnea, diffuse pulmonary infiltrates on chest x-ray, restrictive pulmonary dysfunction, and hypoxemia. This constellation of findings usually erroneously suggests PCP, and a lung biopsy is necessary to establish the diagnosis. Typical microscopic findings include diffuse infiltration of the pulmonary interstitium with a mixture of lymphocytes and plasma cells; immunohistologic studies reveal that in association with HIV infection, these lymphocytes are T cells. The pathogenesis of LIP in patients with HIV infection is not known. It is believed that it represents a tissue response to EBV infection, HIV infection of the lung, or both. Although patients with LIP may respond dramatically to corticosteroid therapy, others may improve with no treatment. Unfortunately, most patients eventually succumb to other complications of HIV infection.