We investigated the clinical features and outcome of 14 patients with anti-myeloperoxidase antibody (MPO-ANCA) positive rapidly progressive glomerulonephritic syndrome. Underlying diseases included microscopic polyarteritis in 6 patients, idiopathic crescentic glomerulonephritis with lung hemorrhage in 2 patients, idiopathic glomerulonephritis in 3 patients, rapidly progressive glomerulonephritic syndrome without renal biopsy in 1 patient, crescentic glomerulonephritis associated with Sjögren syndrome and progressive systemic sclerosis in 1 patient and crescentic glomerulonephritis associated with sarcoidosis in 1 patient. Five patients were male (mean age, 59.2 years) and 9 were female (mean age, 54.0 years). On admission, most patients had anemia, leukocytophilia, and marked elevation of C-reactive protein (CRP). Average hemoglobin, white blood cell count and CRP levels on admission were 8.1 mg/dl, 11,500/mm3 and 14.7 mg/dl, respectively. Average serum creatinine was 4.0 mg/dl. All patients were treated with steroids either with or without cyclophosphamide. As the patients recovered clinically, the MPO-ANCA titers declined. Although most patients responded well to immunosuppressive therapy, some died of serious complications (such as acute respiratory distress syndrome, fungal infection, and pneumocystis pneumonia). The prognosis of patients with severe renal failure was especially poor. We conclude that early diagnosis, treatment and intensive care during immunosuppressive therapy are very important in the management of MPO-ANCA-positive rapidly progressive glomerulonephritis.