We compared the histological changes before and after treatment in 14 cases of myeloperoxidase antineutrophil cytoplasmic autoantibodies (MPO-ANCA) related nephritis in whom we were able to perform two renal biopsies. The results show that the clinical findings and acute glomerular and tubulointerstitial injuries decreased, while chronic glomerular injuries increased. No changes were seen in minor glomerular abnormalities(MGAs) or chronic tubulointerstitial injuries between the first and second biopsies. In the vascular system, no treatment related aggravation of arteriosclerosis occurred and it was found that fibrinoid necrosis disappeared with treatment. Finally, in MPO-ANCA related nephritis, the care given between the first and second biopsies caused acute glomerular injuries to become chronic glomerular injuries, but no changes were detected in the MGA. We believe that the changes in acute tubulointerstitial injuries reflected an improvement in renal function, since the acute tubulointerstitial injuries obviously improved in response to PSL, contributing to the improved renal function. In other words, MPO ANCA-related nephritis is a condition that involves "acute glomerulonephritis+ acute tubulointerstitial nephritis + angiitis," and it is thought that the characteristics of each are independent. We believe that the renal function improved as the acute tubulointerstitial nephritis improved, while the acute glomerular injuries developed into chronic glomerular injuries.