Lung tissue obtained from eight consecutive patients with systemic lupus erythematosus complicated by severe, acute pulmonary disease was studied by both light and immunofluorescence microscopy. Light microscopic examination disclosed interstitial pneumonia in four cases, cytomegalovirus pneumonitis in one case, bronchiolitis and peribronchiolitis in one case, pulmonary infarction in one case and focal atelectasis in the remaining case. Direct immunofluorescence examination revealed focally bound immunoglobulins or complement (C3) within pleural and/or pneumocyte nuclei in each specimen. Immunohistologic studies in these cases may thus suggest a diagnosis of systemic lupus erythematosus with acute pulmonary complications, despite the lack of specificity of the pathologic changes seen by light microscopy.