In order to examine the clinical features of pulmonary involvement in patients with myeloperoxidase specific-antineutrophil cytoplasmic antibody (MPO-ANCA), 46 MPO-ANCA positive patients with collagen-vascular disease and glomerulonephritis were investigated. Twenty eight patients (60.8%) had pulmonary involvement without tumor and infection, 20 (43.5%) of MPO-ANCA positive patients had interstitial pneumonitis, 11 (23.9%) had pulmonary hemorrhage, 3 had asthma (6.5%) and 7 had both interstitial pneumonitis and pulmonary hemorrhage. Patients with pulmonary involvement were older (mean age 61.1) and they had higher inflammatory acute phase reactants and higher mortality (42.9%) than patients without pulmonary involvement. Pulmonary hemorrhage revealed on both lung fields in 10 patients, 7 patients required artificial respirator and 6 died. Titer of MPO-ANCA increased paralleled with progression of pulmonary hemorrhage. Interstitial fibrosis revealed predominantly lower and lateral side of the lung. It is suggested that MPO-ANCA may be closely related to the pathogenesis of pulmonary hemorrhage and interstitial pneumonia. Careful management is needed in patients with pulmonary involvement, especially pulmonary hemorrhage in patients with MPO-ANCA.