The authors present the results of an estimation of the value of the increasing rate in the level of estradiol and in the number of maturing follicles by the end of a gonadotropin stimulation period in the prediction of early and late ovarian hyperstimulation syndrome (OHSS), by examining 141 patients aged 22 to 38 years, who had infertility whose elimination required an in-vitro fertilization procedure. There is evidence that it is expedient to use preventive measures against the risk of the clinical manifestation of early OHSS in all the patients with more than 20 maturing follicles.