Among the various types of primary systemic vasculitides, which were recently newly classified (Chapel Hill Classification, 1994), in two forms of small vessel vasculitis, i.e. in Wegener's granulomatosis and in microscopic polyangiitis, the diagnostic utility of anti-neutrophil cytoplasmic antibodies (ANCA) is now well established. In most cases of Wegener's granulomatosis and microscopic polyangiitis the target antigens of these autoantibodies are proteinase 3 (c-ANCA) or myeloperoxidase (p-ANCA), respectively. Both of these autoantibodies are probably not only diagnostic tools but also contribute to the pathogenic mechanisms causing the vascular damage. In Wegener's granulomatosis and in microscopic polyangiitis the upper and lower respiratory tract and the kidneys are preferentially affected. The lesions of Wegener's granulomatosis show an extremely wide morphologic spectrum. In the oropharynx, nasal sinuses, trachea, and large bronchi, they appear for the most part as ulcerations in which a granulomatous response may or may not be evident. Thus in the absence of granulomas, especially in small biopsy material, the diagnosis of Wegener's granulomatosis should by no means be excluded. In the kidneys a focal and segmental necrotizing glomerulonephritis, often associated with crescent formation, is the typical lesion in Wegener's granulomatosis and in microscopic polyangiitis, as well. Histologically, this glomerulonephritis does not differ from the forms also seen in Schoenlein-Henoch disease, in Goodpasture's syndrome and in some cases of systemic lupus erythematosus, but in Wegener's granulomatosis and in microscopic polyangiitis immune glomerular deposits are uncommon. In fact, by analysis of our biopsy material, these "pauci immune" type of necrotizing glomerulonephritis account for nearly 70%. In the past, Wegener's granulomatosis was usually fatal within 5 months of diagnosis. After introduction of the combined therapy with steroids and cyclophosphamide the prognosis improved considerably. Nevertheless, a further improvement is necessary. This could be reached, without any doubt, by an earlier detection of the disease.