The authors describe the clinical, laboratory and morphological findings, the clinical course and response to treatment in three patients with rapidly progressing glomerulonephritis where positive antibodies against the cytoplasm of neutrophil granulocytes (ANCA) were found. The authors confront their own clinical experience with data in the literature. Examination of ANCA is not only a significant contribution towards a more accurate diagnosis of renal vasculitis, but also an indicator of the activity of the disease and thus of the effectiveness of immunosuppressive treatment.