The MAPK MEK/ERK pathway is often upregulated in cancer cells and represents an attractive target for development of anticancer drugs. Only few data concerning the specific functions of ERK1 and 2 are reported in the literature. In this report, we investigated the specific role of ERK1 and 2 in liver tumor growth both in vitro and in vivo. DNA synthesis and cells in S phase analysed by flow cytometry, correlated with strong inhibition of Cdk1 and cyclin E levels, are strongly reduced after exposure to the MEK inhibitor, U0126. We obtained a significant reduction of colony formation in soft agar assays and a reduction in the size of tumor xenografts in nude mice treated with U0126. Then, we could specifically abolished ERK1 or 2 expression by small-interfering RNA (siRNA) and demonstrated that ERK2 knockdown but not ERK1 interferes with the process of replication. Moreover, we found that colony formation and tumor growth in vivo were significantly inhibited by targeting ERK2 using stable chemically modified siRNA. Taken together, our results emphasize the importance of the MEK/ERK pathway in liver cancer cell growth in vitro and in vivo and argue for a crucial role of ERK2 in this regulation.