Renal involvement in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis is characterized by focal segmental crescentic and/or necrotizing glomerulonephritis. Here, we report the case of a 66-year-old woman showing myeloperoxidase (MPO)-ANCA positivity and mononeuritis multiplex whose kidney biopsy revealed severe and diffuse tubulointerstitial nephritis despite the fact that crescentic necrotizing glomerulonephritis was focal. The mechanism of tubulointerstitial injury in ANCA-associated vasculitis remains unclear. Further studies are necessary to confirm the relationship between diffuse tubulointerstitial nephritis and ANCA-associated vasculitis.