Lung tissue and tissue from the lymphoreticular system obtained at open biopsy and/or autopsy were studied in ten children with the acquired immunodeficiency syndrome (AIDS). One or both parents of nine of the children had AIDS or risk factors for AIDS. The remaining child had hemophilia. The following pulmonary lesions were seen: 1) diffuse alveolar damage (DAD), 2) Pneumocystis carinii and/or cytomegalovirus pneumonitis, 3) lymphoid interstitial pneumonitis (LIP), and 4) desquamative interstitial pneumonitis (DIP). Combinations of such factors as mechanical ventilation, oxygen therapy, and opportunistic infection played a role in the pathogenesis of DAD. Opportunistic infections were related to the defective cell-mediated immunity in these children. The clinical, epidemiologic, immunologic, and pathologic features of the thymuses of these patients indicate that the immune deficiency was unlikely to have been of congenital origin. The immunologic abnormalities may also have been related to the pathogenesis of LIP and DIP. Neither LIP nor DIP has been described in adults with AIDS. Open lung biopsy is of practical importance in the diagnosis and treatment of pulmonary disease in children with AIDS.