In a short-term protocol, the influence of progestogen pretreatment upon the oestradiol flare-up (delta E2) induced by gonadotrophin-releasing hormone agonist (GnRHa) was assessed in relation to in-vitro fertilization (IVF) outcome in 90 cycles programmed (n = 52) or not (n = 38) by norethisterone (10 mg/day for 12-20 days). Patients pretreated by progestogen had a significantly lower delta E2 value than patients without pretreatment (delta E2 = 26 +/- 5 versus 61 +/- 8 pg/ml, P = 0.003). It could be related to a lower gonadotrophic response for luteinizing hormone (LH) (delta LH = 9 +/- 0.8 versus 14.5 +/- 2.2 IU/l, P = 0.01). The IVF outcome (final oestradiol, number of oocytes or embryos) was similar in both groups and delta E2 was well correlated with these final parameters in each group. A significant rise in serum progesterone was observed only in patients without pretreatment (delta P = 1.1 +/- 0.2 versus 0.1 +/- 0.1 ng/ml, P < 0.0001). Thus norethisterone pretreatment decreases the oestradiol flare-up and prevents the early increase of progesterone (by avoiding some rescue of the corpus luteum or some luteinization of small developing follicles) but does not influence the outcome of the IVF cycle. In clinical practice, evaluation of the hormonal flare-up for predicting IVF outcome must take into account any pretreatment prescription.