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Purpose: The aim of this study is to investigate the mechanism of long-term acceptance of ABO-incompatible renal allograft. Methods: Thirty-four ABO-incompatible kidney transplant recipients with blood group O were entered in this study. Eighteen and 16 recipients received a blood group B (B-to-O) and A1 (A1-to-O) allograft, respectively. Sixteen (O-to-O) recipients were control subjects. The titers of blood group antibodies were measured before pre-conditioning and following transplantation. Blood group antigens were stained in 32 graft biopsy specimen. Results: In 18 (B-to-O) and 16 (A1-to-O) recipients, only anti-B and anti-A1 antibodies, respectively, were significantly suppressed compared with those in 16 (O-to-O) more than 1 year post-transplantation. In eight (B-to-O) and seven (A1-to-O) recipients, only anti-B and anti-A1 antibodies, respectively, were significantly suppressed 1 year post-transplantation, compared with those pre-conditioning. Blood group antigens were stained with same intensity and pattern in any biopsy specimen under all conditions. There was no significant deposition of IgG and IgM in any biopsy specimen. These suggested that production of anti-donor blood group antibodies appeared to be specifically suppressed in long-term stable ABO-incompatible kidney transplant recipients. Conclusion: Accommodation of ABO-incompatible kidney transplantation could be explained by donor-specific blood group antibody suppression.