Induction of olfactomedin 4 (OLFM4(GW112/hGC-1)) in cancer cells was recently reported to have a novel antiapoptotic action via binding to the potent apoptosis inducer GRIM-19. We sought to clarify undiscovered functions of constitutively expressed OLFM4 in cancer cells. OLFM4 mRNA was highly expressed in pancreatic cancer tissues. In PANC-1 cell cultures, expression was especially elevated during early S phase of the cell cycle. Transduction of small interfering RNA for OLFM4 to decrease mRNA expression caused time-dependent growth inhibition, with typical early S-phase arrest after 6 days. In addition, cell volume increased without increases in multinucleated cells, consistent with premitotic inhibition of DNA synthesis. Inhibition of OLFM4 mRNA expression by small interfering RNA did not promote apoptosis. Taken together, the results indicate that OLFM4 promotes proliferation of PANC-1 cells by favoring transition from the S to G(2)/M phase.