Graft-versus-host disease (GVHD) remains the major complication after allogeneic bone marrow transplantation (BMT). The process whereby acute GVHD mediated by alloreactive donor T cells transitions into chronic GVHD, which is characterized by prominent features of auto-immunity, has long been unresolved. In this study, we demonstrate that GVHD-associated autoimmunity and, by extension, chronic GVHD is attributable to the progressive loss of CD4⁺CD25⁺Foxp3⁺ regulatory T cells during the course of acute GVHD. This leads to the expansion of donor-derived CD4⁺ T cells with T_H1 and T_H17 cytokine phenotypes that release proinflammatory cytokines and cause autoimmune-mediated pathological damage. These T cells are present early after transplantation, indicating that the pathophysiological events that lead to chronic GVHD are set in motion during the acute phase of GVHD. We conclude that the absence of CD4⁺CD25⁺ regulatory T cells coupled with unregulated T_H1 and T_H17 cells leads to the development of autoimmunity and that donor-derived T_H1 and T_H17 cells serve as the nexus between acute and chronic GVHD.