Amplification of chromosomal DNA is thought to be one of the mechanisms activating cancer-related genes in tumors. To identify the most likely target for amplification in the region 19q13.12-q13.2, detected previously in SKN-3 cells by a genome-wide screening of DNA copy-number aberrations in a panel of oral squamous-cell carcinoma (OSCC) cell lines, we determined the extent of the amplicon, analyzed a panel of cell lines for the expression of candidate genes within the amplicon, and then evaluated growth-suppressive effects by knocking down genes of interest. Reported information about the function and/or expression of each gene, remarkable overexpression in SKN-3 cells and relatively frequent overexpression in additional OSCC lines compared with an immortalized normal oral epithelial cell line, and expression level-dependent proliferation-promoting activity led us to conclude that the p21-activated kinase 4 (PAK4) gene was the most likely target. An immunohistochemical analysis of primary tumors from 105 cases of head and neck SCC including 50 cases of OSCC demonstrated the overexpression of PAK4 to be significantly associated with a poorer prognosis. These findings reveal that the PAK4 overexpression through amplification or other mechanisms promotes the proliferation and/or survival of OSCC cells, and that PAK4 might be a good diagnostic and/or therapeutic target.