A patient with systemic lupus erythematosus (LE) was admitted with an acute illness characterized by dyspnea, hemoptysis, fever, and diffuse infiltrates revealed by chest roentgenograms. An open-lung biopsy specimen showed massive intrapulmonary hemorrhage, and immunofluorescence and electron microscopic examinations showed granular deposits of IgG within the alveolar walls and pulmonary vessels. Rare, widely scattered foci of infiltration of vessel and alveolar walls by polymorphonuclear leukocytes were also observed. The patient was treated with large doses of prednisone and her condition improved, but she had a second episode of massive hemorrhage eight days after discharge despite maintenance of the prednisone therapy. Her condition again improved over several days, and she has had no pulmonary symptoms for nine months. In some instances, pulmonary hemorrhage in patients with systemic LE may be mediated by polymorphonuclear leukocytes attracted by immune-complex deposits.