The coinhibitory receptor programmed death-1 (PD-1) maintains immune homeostasis by negatively regulating T cell function and survival. Blockade of PD-1 increases the severity of graft-versus-host disease (GVHD), but the interplay between PD-1 inhibition and T cell metabolism is not well studied. We found that both murine and human alloreactive T cells concomitantly upregulated PD-1 expression and increased levels of reactive oxygen species (ROS) following allogeneic bone marrow transplantation. This PD-1 Hi ROS Hi phenotype was specific to alloreactive T cells and was not observed in syngeneic T cells during homeostatic proliferation. Blockade of PD-1 signaling decreased both mitochondrial H 2 O 2 and total cellular ROS levels, and PD-1–driven increases in ROS were dependent upon the oxidation of fatty acids, because treatment with etomoxir nullified changes in ROS levels following PD-1 blockade. Downstream of PD-1, elevated ROS levels impaired T cell survival in a process reversed by antioxidants. Furthermore, PD-1–driven changes in ROS were fundamental to establishing a cell’s susceptibility to subsequent metabolic inhibition, because blockade of PD-1 decreased the efficacy of later F 1 F 0-ATP synthase modulation. These data indicate that PD-1 facilitates apoptosis in alloreactive T cells by increasing ROS in a process dependent upon the oxidation of fat. In addition, blockade of PD-1 undermines the potential for subsequent metabolic inhibition, an important consideration given the increasing use of anti–PD-1 therapies in the clinic.