It has long been known that endometrial regeneration and proliferation is regulated by sex steroids, cytokines and various growth factors. The mechanisms responsible for such organized growth are still under investigation. Human chorionic gonadotropin/luteinizing hormone (hCG/LH) receptors have been found to be localized in human endometrium by immunocytochemistry. Gonadotropins have been widely used for hyperstimulation during in vitro fertilization (IVF) procedures; however, the direct effect of gonadotropin on the endometrium has not been adequately investigated yet. This study attempted to define the effect of gonadotropins on the proliferation of human endometrial stromal cells in vitro. Human endometrial stromal cells were obtained from hysterectomy specimens and cultured in serum-containing media for up to 72 h. The effects of adding 7.5, 15, 30, 150 mIU/ml of human menopausal gonadotropin (hMG) and follicle stimulating hormone (FSH), and 10, 100, 1,000, 10,000 mIU/ml of hCG on cumulative [(3)H]-thymidine incorporation in endometrial stromal cells were assessed. This study demonstrated that FSH and hMG induced significant inhibition in [(3)H]-thymidine uptake at all concentrations, respectively (P<0.05). In contrast to the above two hormones, hCG exerted inhibitory effect at concentrations of 1,000 and 10,000 mIU/ml (P<0.05). There was no evidence of dose-response correlations in all three gonadotropin experiments. These data imply that gonadotropins at the concentrations studied inhibit the proliferation of human endometrial stromal cells, at least, in short-term culture in vitro. Accordingly, we cannot negate the possibility that administered gonadotropin during ovarian hyperstimulation may directly influence the proliferation of human endometrial cells.