The effect of follicular aspiration and oocyte retrieval on hormonal parameters was examined in women undergoing ovarian stimulation for in-vitro fertilization (IVF) compared to induced ovulation in women undergoing ovarian stimulation for intrauterine insemination (IUI). Blood samples were collected immediately before and 1 h after oocyte retrieval and 48 h later on the day of embryo transfer in 25 IVF patients and before the insemination and 48 h later in 20 IUI patients. A highly significant fall in serum levels of oestradiol (E2), progesterone (P) and human chorionic gonadotrophin (HCG), (P less than 0.001) was observed in the IVF group 1 h after follicular aspiration. The decline in serum E2 levels was maintained at 48 h. In contrast, there was no significant change in serum E2 levels in the IUI group during 48 h. The immediate decline in E2 levels after follicular aspiration might play a role in preventing ovarian hyperstimulation syndrome.