Objective To investigate the pathogenesis of diseases with antineutrophil cytoplasmic antibody (ANCA)and its diagnostic value in primary small vessel vasculitis.Methods 57 patients with serum ANCA positive were involved in this study,and ELISA was employed to assay anti-MPO.Patients with ANCA positive,anti-MPO positive and/or anti-PR3 positive were involved in group A.Patients with ANCA positive,anti-MPO negative and anti-PR3 negative were involved in group B.X2 was used to analyze the differences between the two groups.Results The etiology of 57 ANCA positive patients included primary small vessel vasculitis (20 cases,35.1%),non inflam-matory connective tissue disease(19 cases,33.3%),non connective tissue disease(18 cases,31.6%).A group of primary small vessel vasculitis accounted for 58.6%,which was significantly higher than 10.7% of the B group (χ2 =14.354,P<0.01);while the B group of non inflammatory connective tissue disease accounted for 50%,which was significantly higher than 17.2%of the A group (χ2 =6.879,P<0.01).Conclusion ANCA should be found in many kinds of diseases,so combined detection of anti-MPO and anti-PR3 should be employed to improve the diag-nosis specificity in primary small vessel vasculitis.Furthermore,non vessel vasculitis connective tissue disease should be excluded in patients with ANCA positive,anti-MPO and anti-PR3 negative.