Purpose: Hepatocellular carcinoma (HCC) is an aggressive malignancy and is a devastating clinical complication of chronic liver disease. Therapeutic options are limited mainly because the genetic and biochemical understanding of this disease remains fragmented.We intended to study the role of signal transducer and activator of transcription 3 (STAT3) aberrant signaling in HCC malignancy, and the therapeutic potential of inhibition of STAT3 expression for HCC. Experimental Design: A 2 ¶-O-methoxyethylribose^modified phosphorothioate antisense oligonucleotide (ASO) was used to knock down STAT3 expression in different human HCC cell lines, including the highly metastatic HCCLM3 derived from orthotopic implantation and subsequent lung metastasis in athymic mice. The effects of STAT3 ASO treatment on HCC cells, metastasis, and animal survival following HCCLM3 orthotopic implantation were evaluated. Results: Specific suppression of phosphorylated STAT3 reduced its DNA-binding activity, inhibited the expression of vascular endothelial growth factor, survivin, matrix metalloproteinases 2 and 9, reduced cell proliferation and migratory potential, induced apoptosis in vitro, and inhibited intradermal angiogenesis and s.c. tumorigenesis upon injection in mice. In mice bearing orthotopically implanted HCCLM3, STAT3 inhibition following therapeutic treatment with STAT3 ASO reduced circulating vascular endothelial growth factor and basic fibroblast growth factor, decreased intratumor CD34-positive microvessel density, intrahepatic and intraperitoneal transmission, and lung metastasis. HCC tumor volume and weight were reduced and the survival time of mice bearing orthotopically xenografted HCC was approximately doubled in STAT3 ASO^treated mice (P < 0.05).Conclusions: Constitutively activated STAT3 is essential for the growth, survival, and metastasis of HCC, suggesting that STAT3-targeted therapy may have utility for HCC.