Unraveling the events that control the suppressive function of regulatory T (Treg) cells is extremely important because it will enable investigators to manipulate these cells to inhibit or enhance their functions as necessary. One of the members of the Signal Transducer and Activators of Transcription (STATs) family, STAT3, has emerged as a negative regulator of inflammatory responses. Here, we study the role of STAT3 in Treg cell induction. We found that GAD-IgG-transduced splenocytes induce a CD4(+)Foxp3(+)Treg cell increase in NOD mice. In parallel with the Treg cell increase, an IL-6-STAT3 signal pathway is activated. When STAT3 activation is blocked, GAD-specific tolerance disappears, the percentage of Treg cells decreases and IL-10 secretion is reduced in the splenocytes of NOD mice recipients of GAD-IgG-transduced splenocytes. Our findings indicate that transcription factor STAT3 plays an important role in immune tolerance.