The transcription factor signal transducer and activator of transcription 3 (Stat3) is constitutively activated in a variety of cancers including squamous cell carcinoma of the head and neck (SCCHN). Previous investigations have demonstrated that activated Stat3 contributes to a loss of growth control and transformation. To investigate the therapeutic potential of blocking Stat3 in cancer cells, we developed a transcription factor decoy to selectively abrogate activated Stat3. The Stat3 decoy was composed of a 15-mer double-stranded oligonucleotide, which corresponded closely to the Stat3 response element within the c-fos promoter. The Stat3 decoy bound specifically to activated Stat3 and blocked binding of Stat3 to a radiolabeled Stat3 binding element. By contrast, a mutated version of the decoy that differed by only a single base pair did not bind the activated Stat3 protein. Treatment of head and neck cancer cells with the Stat3 decoy inhibited proliferation and Stat3-mediated gene expression, but did not decrease the proliferation of normal oral keratinocytes. Thus, disruption of activated Stat3 by using a transcription factor decoy approach may serve as a novel therapeutic strategy for cancers characterized by constitutive Stat3 activation.