The clinicopathological features of four patients with systemic lupus erythematosus and pulmonary hemorrhage are described. Our study confirms that pulmonary hemorrhage may be a dominant clinical expression of lung involvement in this disease. Its clinical manifestations are usually quite characteristic. However, hemoptysis may be absent. Radiographically, bilateral alveolar infiltrates resembling pulmonary edema or infection may be seen. Pulmonary hemorrhage was a major contributing factor to the death of three of our patients. The possible pathogenetic mechanisms responsible for pulmonary hemorrhage in our patients and other patients previously recorded in the literature are reviewed. Evidence supporting an immune complex pathogenesis is presented. Our immunopathological and ultrastructural studies demonstrate deposition of immune aggregates in the lungs in the alveolar septa, large blood vessels, and bronchioles in a manner similar to that which has been observed in the experimental serum sickness model of immune complex mediated pulmonary injury. The histological abnormalities, although nonspecific, are consistent with this interpretation, and collectively show diffuse alveolar lining cell and endothelial cell injury. However, an immune complex pathogenesis may not completely explain the occurrence of pulmonary hemorrhage in SLE. Other factors, including bleeding disorders, pulmonary infection, oxygen toxicity, and the "shock lung" syndrome, may also have contributed to lung hemorrhage in some of these patients.