The S-phase kinase-associated protein-2 (SKP2) plays a key role in ubiquitin-mediated proteolysis, which results in the progression of cells from a quiescence to proliferative state. SKP2 is overexpressed in a variety of tumors. In this study, we used small interfering RNAs (siRNAs) to inhibit the SKP2 expression in lung cancer cells and thereby investigate the role of SKP2 in lung tumorigenesis. Three lung cancer cell lines were transfected with siRNAs targeted against SKP2. SKP2-siRNAs specifically and efficiently reduced the levels of the SKP2 protein by 90% 48 h after transfection in all cell lines. In the A549 and H1792 cells, p27 expression was increased and the increase was inversely proportional to the level of SKP2; cell proliferation was reduced to 12 and 28%, respectively; apoptosis was increased to 36 and 30%, respectively; 36 and 28% of cells accumulated in the sub-G1 phase, respectively; and the population of cells in the G1 phase was decreased to 37 and 41%, respectively. In addition, the SKP2-depleted A549 and H1792 cells showed decreased levels of cyclin E/CDK2. Correspondingly, only 4 and 6% of the treated A549 and H1792 cells had multiple centrosomes, respectively, compared with 43 and 46% of the control cells, respectively. These results imply that SKP2 plays an oncogenic role in lung cancer and that SKP2 silencing may be useful in the treatment of lung cancer.