Reactive oxygen species (ROS) promote tumor cell proliferation and survival by directly modulating growthregulatory molecules and key transcription factors. The signal transducer and activator of transcription 3 (STAT3) is constitutively active in a variety of tumor cell types, where the effect of ROS on the Janus kinase/STAT pathway has been examined. We report here that STAT3 is directly sensitive to intracellular oxidants. Oxidation of conserved cysteines by peroxide decreased STAT3 binding to consensus serum-inducible elements (SIE) in vitro and in vivo and diminished interleukin (IL)-6-mediated reporter expression. Inhibitory effects produced by cysteine oxidation in STAT3 were negated in redox-insensitive STAT3 mutants. In contrast, ROS had no effect on IL-6-induced STAT3 recruitment to the c-myc P2 promoter. Expression of a redoxinsensitive STAT3 in breast carcinoma cells accelerated their proliferation while reducing resistance to oxidative stress. Our results implicate STAT3 in coupling intracellular redox homeostasis to cell proliferation and survival. ©2010 AACR.