Tetraspanin CD151 is involved in several pathological activities associated with tumor progression, including neoangiogenesis. However, the role and molecular mechanism of CD151 in the neoangiogenesis of hepatocellular carcinoma (HCC) remain enigmatic. We found that the level of expression of matrix metalloproteinase 9 (MMP9) was positively associated with CD151 expression in HCC cells. We developed a zone-by-zone blockade and demonstrated that overexpression of CD151 in HCC cells facilitated MMP9 expression through a phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/glycogen synthase kinase 3β (GSK-3β)/Snail signaling pathway. In contrast, down-regulation of CD151 expression impaired the ability of HCC cells to form microvessels in vitro and reduced their in vivo metastatic potential. In a clinical setting, a significant correlation of the expression of CD151 with MMP9 expression and with microvessel density (MVD) was revealed by Pearson correlation analysis of HCC patients. The postoperative 3-, 5-, and 7-year overall survival rates of HCC patients with CD151^high/MMP9 ^high/MVD^high were significantly lower than those of the CD151^low/MMP9^low/MVD^low group or groups in which only one or two of CD151, MMP9, and MVD were highly expressed. Cumulative recurrence rates were also highest in HCC patients with CD151 ^high/MMP9^high/MVD^high in comparison with the other groups. Multivariate Cox proportional hazards analysis showed that the concomitant overexpression of CD151, MMP9, and MVD was an independent marker for predicting poor prognosis of HCC. Conclusion: Overexpression of CD151 up-regulated the expression of MMP9 through the PI3K/Akt/GSK-3β/Snail pathway. CD151-dependent neoangiogenesis appeared to promote the progression of HCC, and this suggests that CD151 may be useful as a high-priority therapeutic target for antiangiogenesis in HCC. Copyright © 2010 by the American Association for the Study of Liver Diseases.