Double unit cord blood transplantation (DUCBT) has emerged as a successful strategy to improve engraftment and decrease transplant related mortality in adults and large children undergoing cord transplantation. In the vast majority of cases, one unit emerges as the sole source of long term hematopoiesis in the recipient following DUCBT. No factors have been identified that reliably predict which unit will emerge as the dominant unit, and limited studies have examined the mechanism underlying the observation. In a recent publication in Blood, we provide the first compelling data that effector CD8(+) T cells play a critical role in the dominant unit actively rejecting the losing unit. Our findings provide an important first step in understanding the interactions following DUCBT, and provide insights that might be used to optimize graft versus leukemia effect and cord unit selection as well as better understand mechanisms of tolerance.