Two cases of anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitis (ANCA-V) occurred in the transplanted kidney were reported. Case 1 was a 57 yr-old female whose original disease was MPO-ANCA-V. A relapse of necrotizing crescentic glomerulonephritis occurred one year after transplantation with positive serum reaction for MPO-ANCA. In spite of several immunosuppressive treatments, the disease progressed and she returned to hemodialysis treatment three yr and seven months after transplantation. Case 2 was a 34 yr-old female whose original disease was IgA nephropathy. She had a stable clinical condition during 13 yr after transplantation; however, de novo onset of necrotizing crescentic glomerulonephritis occurred at 14 yr 10 months after transplantation with positive serum reaction for MPO-ANCA. She returned to hemodialysis treatment five yr after the onset of ANCA-V. Urinary abnormities such as microhematuria and proteinuria were useful diagnostic findings but the titers of serum MPO-ANCA were relatively low in both patients. Concerning the treatment, steroid pulse therapy was effective in some extents but the disease progressed to graft failure in both cases. ANCA-V is a severe glomerulonephritis which can occur in kidney allograft in the manner of relapse and de novo. Detection of urinary abnormalities and positive serum ANCA combined with histological confirmation of necrotizing crescentic glomerulonephritis and/or vasculitis is required for early diagnosis and effective treatment of ANCA-V in renal transplant patients.