The present study attempted to define the clinical, radiological, immunological and pathological characteristics of microscopic polyangiitis (MPA), and to separate them from classic PAN (c-PAN) and Churg-Strauss syndrome (CSS). In most cases, patients presenting microaneurysms and/or multiple vessel stenoses, which reflect medium-sized vessel involvement, did not have antineutrophil cytoplasmic antibodies (ANCA) (6.6%). Conversely, patients with glomerulonephritis almost never had abnormal angiograms. Furthermore, the clinical characteristics of ANCA-positive patients also indicate small-sized vessel involvement. Skin involvement (73.1 vs 26.7%, P < or = 0.05), glomerulonephritis (38.5 vs 0%, P < or = 0.001) and the presence of ANCA (34.6 vs 6.7%, P < or = 0.05) were significantly more frequent in patients with normal than abnormal angiograms, respectively. Conversely, hypertension (66.7 vs 23.1%, P < or = 0.02), renal vasculitis (46.7 vs 0%, P < or = 0.001) and hepatitis B antigenaemia (60 vs 11.5%, P < or = 0.01) were significantly more common in patients with abnormal angiograms. Stratification of patients according to vessel size showed that, except for skin involvement (P < or = 0.05) and glomerulonephritis (P < or = 0.01), which are direct manifestations of small-sized vessel diseases, clinical symptoms of PAN or CSS, angiographic findings and ANCA were not correlated to arteriole size. Although at present it is not possible to separate definitively MPA from c-PAN, our results show that ANCA should be considered diagnostic for MPA and, in most cases, should be an exclusion criterion for c-PAN. Conversely, small-sized vessel involvement can be observed in patients presenting characteristics of c-PAN, MPA or CSS and, therefore, is not a sufficient criterion for assigning diagnosis.