OBJECTIVE To find out the differences between human granulosa-luteal cells derived from natural and stimulated IVF-ET cycles. METHODS Cells were obtained from dominant follicles of 52 women with natural cycles in whom preovulatory hCG was given when the follicle was mature and from 50 dominant follicles of women undergoing IVF-ET with hMG and hCG. METHODS In Vitro Fertilization Unit, University Department of Obstetrics and Gynecology Ljubljana, Slovenia. METHODS Four subpopulations of cells were observed by computerized image analysis in which hCG was localized using immunoperoxidase staining. RESULTS The nonluteinized granulosa cells from natural cycles were larger than those from the stimulated ones. In luteinized cell types, there was no difference in cell area between natural and stimulated cycles. Three staining types of hCG localization were found: on the surface membrane, on the surface membrane and within the cytoplasm, and within the cytoplasm alone. The hCG stained cells from natural cycles were larger than the ones from stimulated cycles. The natural developing follicles contained more hCG stained cells than the stimulated ones. The follicles with fertilizable oocytes had more cells with cytoplasmic hCG localization. Only in natural cycles was there was a correlation between follicular fluid hCG levels and the percentage of the hCG stained cells. CONCLUSIONS We found differences in morphometric characteristics and hCG localization between human granulosa and granulosa-luteal cells obtained from natural and stimulated IVF-ET cycles.