Follicle monitoring in the normal and clomiphene·stimulated cycles were analyzed in the Seoul IVF and ET (In vitro fertilization and embryo transfer) program. Ovarian follicular diameters were measured by the real·time sector scanner and plasma estradiol levels were assayed by radioimmunoassay methods during periovulatory period. The maximum follicular sizes of the clomiphene-stimulated and normal cycles were 21.1+-3.4mm and 19.2+-0.8mm, respectively. The peak levels of plasma estradiol in the clomiphene-stimulated and normal cycles were 10538+-553.6ng/ml and 298.3+-39.6pg/ml, respectively. Daily growth rate of the follicular diameters of the clomiphene-stimulated and normal cycles were 2.1mm and 1.9mm, respectively. Mean follicular number of the clomiphene-simulated and normal cycles were 2.28+-1.12 and 1.12+-0.21, respectively. There was a good statistical correlation between the mean follicular diameters and the plasma estradiol levels in the normal ovulatory and c1omiphene-stimulated ovulatory menstrual cycles (p<0.05). Our data revealad that the mean follicular diameter and the plasma estradiol level prior to HCG administration in IVF and program should reach at the level of 17.8+-3.0mm and 949.4+-487.1 pg/ml, respectively.