<p><b>OBJECTIVE</b>To explore the effects of mifepristone on the growth of human gastric cancer cell line MKN-45 and its possible mechanisms.</p><p><b>METHODS</b>In situ hybridization was used to detect the expression of progesterone receptor (PR) mRNA in MKN-45 cells. Proliferation, cell cycle distribution, and the expression of Bcl-xL and vascular endothelial growth factor (VEGF) of MKN-45 cells incubated with various concentrations of mifepristone (1, 5, 10, and 20 micromol/L) were analyzed using MTT reduction assay, flow cytometry, reverse transcription-polymerase chain reaction (RT-PCR), and enzyme-linked immunoabsorbent assay (ELISA), respectively. After transplantation of MKN-45 cells underneath the skin of athymic mice, mifepristone was administrated with the dose of 50 mg/(kg x d) for 6 weeks to evaluate the tumor growth. Apoptosis and the expression of proliferating cell nuclear antigen (PCNA) in xenografted tumors were detected using transmission electron microscopy and immunohistochemical staining, respectively.</p><p><b>RESULTS</b>PR mRNA was highly expressed in cultured MKN-45 cell. Mifepristone dose-dependently inhibited the proliferation of MKN-45 cells, and the inhibitory rate was dramatically increased from 7.21% to 47.23%. The inhibitory effect was accompanied by a dose-dependent increase in the percentage of cells in G0/G1 phase, and with a concurrent decrease in the proportion of S- and G2M-phase cells and the proliferative index from 57.65% to 24.54%. Meanwhile, mifepristone down-regulated the expression of Bcl-xL and VEGF in a dose-dependent manner. In vivo, mifepristone effectively inhibited the growth of xenografted tumors in nude mice (55.14% for inhibitory rate), induced apoptosis, and down-regulated PCNA expression in gastric cancer.</p><p><b>CONCLUSION</b>Mifepristone exerts significant growth inhibitory effects on PR-positive human MKN-45 gastric cancer cells via multiple mechanisms, and may be a beneficial agent against the tumor.</p>