BACKGROUND: No data are currently available regarding kinetics of human endometrial steroid receptors in stimulated cycles.METHODS: In 31 patients (age <39 years) stimulated with gonadotrophins and GnRH antagonists for intrauterine insemination (IUI) an endometrial biopsy was performed on the first day after the end of menstruation and a second biopsy was performed two (Group 0 + 2, n = 10) or four (Group 0 + 4, n = 11) days after the first biopsy, or on the day of hCG administration (Group 0 + hCG, n = 10). Expression of progesterone (PR) and estrogen (ER) receptor was investigated by immunohistochemistry using monoclonal antibodies.RESULTS: PR and ER levels were significantly increased in the second versus the first biopsy, in all groups analyzed (P = 0.01), in both stromal and glandular cells. Between the three groups compared, a significant increase in PR expression was observed for glandular cells (P = 0.03), with the highest value observed in Group 0 + 4. Moreover, the increase in PR expression in stromal cells differed between groups (P = 0.01), with the highest value observed in the Group 0 + hCG.CONCLUSIONS: In stimulated cycles for IUI, ER expression in both glandular and stromal endometrial cells, after an initial increase, does not appear to change significantly during the follicular phase. On the contrary, during the same period of time, following an initial rise, PR expression in glandular and stromal cells continues to increase.