ABSTRACT.Purpose: To evaluate effects of ABO antigen blood-group matching on the rates of corneal allograft rejection after penetrating keratoplasty. Methods: We retrospectively studied clinical results of penetrating keratoplasties in terms of graft survival rates and rejection-free graft survival rates. Penetrating keratoplasties were done on 95 eyes between 1993 and 1997. Clinical results were analyzed using the Kaplan-Meier life table method and log-rank test. The corneal transplantations were classified into 2 groups, high-risk (35 eyes) and low-risk (60 eyes) transplantations. High-risk transplantation was defined as significant vascularization in the recipient corneas or a history of graft failure. The remaining transplantations were defined as low-risk. Results: The respective graft survival (pΩ0.031) and rejection-free graft survival (pΩ0.0097) rates were higher in the low-risk than in the high-risk group. In the high-risk group, the rejection-free graft survival was 68.9% for the ABOcompatible subgroup and 36.4% for the ABO-incompatible subgroup (pΩ0.007). Conclusion: ABO matching is effective in reducing the risk of allograft rejection in high-risk corneal transplantations.