Antigen-presenting cells (APCs) can induce T cell activation as well as T cell tolerance. The molecular mechanisms by which APCs regulate this critical decision of the immune system are not well understood. Here we show that Stat3 signaling plays a critical role in the induction of antigen-specific T cell tolerance. Targeted disruption of Stat3 signaling in APCs resulted in priming of antigen-specific CD4⁺ T cells in response to an otherwise tolerogenic stimulus in vivo. Furthermore, APCs devoid of Stat3 effectively break antigen-specific T cell anergy in vitro. Conversely, increased Stat3 activity in APCs led to impaired antigen-specific T cell responses. Stat3 signaling provides, therefore, a novel molecular target for manipulation of immune activation/tolerance, a central decision with profound implications in autoimmunity, transplantation, and cancer immunotherapy.