Endothelial cells serve an important role in augmenting immune responses through enhanced expression of MHC class II antigens. Immune-mediated vascular injury associated with rejection requires reendothelialization to restore vascular integrity. The origin of the reparative endothelial cells can be determined when ABO antigens expressed on these cells differ in the donor and recipient. To assess the frequency and significance of reendothelialization by recipient endothelial cells, we stained serial endomyocardial biopsies for ABO antigens in 34 (13%) compatible, nonidentical cardiac allograft recipients of 268 cardiac transplant procedures. In ten (30%) the allograft endothelial cells expressed the characteristics of the recipient (five partial and five complete) within 7.5 +/- 1.0 months (mean +/- SEM) after transplantation. Over 26.3 +/- 2.5 months follow-up no differences could be detected in pretransplant characteristics, allograft survival, survivor rejection morbidity, long-term allograft function, and presence of coronary vasculopathy between those whose endothelial cells expressed recipient blood group antigens and those who did not, which may merely be a reflection of the small sample size. This study indicates that recipient reendothelialization occurs frequently following cardiac transplantation and may result from immune-mediated vascular injury. The effect of recipient reendothelialization on allograft tolerance requires further investigation.