GVHD is a major barrier to broader use of allogenic HSCT for nonmalignancy clinical applications such as the treatment of primary immunodeficiencies and hemoglobinopathies. We show in a murine model of C57BL/6J (H2-k(b)) --> B6D2F1/J (H2-k(b/d)) acute GVHD that when initiated 2 days before transplant, the activation of the adenosine A(2A)R with the selective agonist ATL146e inhibits the weight loss and mortality associated with disease progression. Furthermore, circulating levels of proinflammatory cytokines and chemokines, including IFN-gamma, IL-6, CCL2, KC, and G-CSF, are reduced significantly by 14-day ATL146e treatment. The up-regulation of CD25, CD69, and CD40L expression by donor CD4(+) and CD8(+) T cells is inhibited by A(2A)R activation; fewer CD3(+) T cells are found in the liver, skin, and colon of ATL146e-treated mice as compared with vehicle-treated controls; and associated tissue injury is lessened. The delayed administration of ATL146e, beginning 9 days after HSCT, reverses GVHD-associated body weight loss successfully, and improvement is sustained for the duration of treatment. We conclude that the selective activation of the A(2A)R has therapeutic potential in the prevention and treatment of acute GVHD.