The authors describe four patients with systemic lupus erythematosus (SLE) and massive pulmonary hemorrhage in whom open-lung biopsies showed a distinctive small-vessel vasculitis. This lesion is characterized by acute inflammation and necrosis involving capillaries, arterioles, and small muscular arteries and is termed microangiitis to reflect the small size of the affected vessels. The involvement of capillaries is manifested by an infiltrate of necrotic neutrophils within alveolar septa often associated with destruction of the alveolar wall. This capillaritis was present in all cases, while involvement of arterioles and small arteries was seen in three. Immunofluorescence and electron microscopy demonstrated immune complexes in only two. The finding of acute microangiitis in a lung biopsy from a patient with pulmonary hemorrhage should suggest the diagnosis of SLE, and it may be a more reliable diagnostic feature than the demonstration of immune complexes.