Regulatory T cells (Tregs) are committed to suppressive functions. Recently, it was proposed that Tregs could produce IL-17 under proinflammatory, polarizing conditions. We studied the role of Tregs on IL-17 production in the absence of exogenous cytokines and insults. Using in vitro and in vivo approaches, we determined that under neutral conditions, simultaneous activation of Tregs and naive CD4+ conventional T cells in the presence of APCs resulted in conversion of Tregs into IL-17–producing cells, and endogenous IL-1β was mandatory in this process. Mechanistic analysis revealed that the IL-1R1 was highly expressed on Tregs and that IL-1β induced marked activation of p38 and JNK, which were involved in IL-17 production. These observations could have important implications on therapeutic strategies using Tregs.