BACKGROUND Blood group O individuals are known to have larger amounts of anti-ABO blood group (anti-A/B) IgG antibodies than A or B individuals. Therefore, in ABO-incompatible (ABOI) kidney transplantation (KTX), it is expected that blood group O recipients are more likely to suffer graft damage, because anti-A/B IgG antibodies are believed to be responsible for worse graft outcomes. METHODS This study assessed the graft outcomes between blood group O and non-O recipients in ABOI-KTX. A total of 164 consecutive recipients who underwent ABOI-KTX between 1990 and 2007 under three different immunosuppressive protocols were enrolled in this study. The study population was divided into two groups: (i) recipients with blood group O (n=87) and (ii) recipients with blood group A or B (non-O) (n=77). RESULTS High anti-A/B IgG titers were predominant in the O group (P<0.001), whereas no significant difference was observed in the IgM titers. The overall graft survival rate did not differ between the two groups; however, the 6-month graft survival rate was significantly lower in the O group (86% vs. 97%, P=0.011). Among 14 recipients who suffered graft loss within 6 months after transplantation, 12 (86%) were O recipients. The cumulative incidence of acute antibody-mediated rejection was significantly higher in the O group (60 days, 31 vs. 14%, P=0.013). CONCLUSIONS Our results may indicate that being a blood group O recipient is at great risk for experiencing early allograft deterioration, probably caused by anti-A/B IgG antibodies in ABOI-KTX.