Cytologic parameters of the vaginal epithelium in 59 women undergoing in vitro fertilization-embryo transfer (ET) were correlated with plasma levels of 17-beta estradiol and progesterone on the oocyte pickup (OPU) and ET days. On the OPU day a late proliferative/late proliferative-early secretory pattern was observed in 84.6% of the 13 women who conceived (group 1) but in only 52% of the 46 who failed to conceive (group 2) (P less than .04). On the ET day a secretory phase appeared in 100% and 61% of group 1 and 2 patients, respectively (P less than .01). Between the OPU and ET days the mean karyopyknotic index dropped by 60% and 13% in group 1 and 2 patients, respectively (P less than .001). It seems that a high estrogenic effect followed by an advanced progestational environment, on the OPU and ET days, respectively, can provide a favorable milieu for implantation.