Introduction. Cytochrome P450 aromatase is a key enzyme in the biosynthesis of estrogens from androgens. Some gynecological diseases are accompanied with changes in the activity of ovarian aromatase. Moreover, changes in the activity of ovarian aromatase are associated with the response to standard gonadotrophin stimulation in women undergoing in vitro fertilization. A new effective method determining the activity of ovarian aromatase without using ovarian tissue biopsy has been developed.The purpose of the present study was to compare the detection of ovarian aromatase activity using the ratio of estradiol to AMH on the second day of the menstrual cycle through the immunocytochemical evaluation of aromatase expression in cumulus cells.Methods. Depending on the activity of the ovarian aromatase, calculated on the second day of the menstrual cycle, 16 patients undergoing IVF protocols were categorized into three groups: eight patients with low activity, four with normal activity, and four with high activity. The patients did not differ in age and BMI. The expression of aromatase in cumulus cells obtained through transvaginal puncture was investigated by the immunocytochemical method.Results. The fluorescence intensities in the low, normal, and high activity groups were 134 (122-161), 238 (201-251), and 259 (240-299), respectively. Differences between these groups were statistically significant (p = 0.008). There was a pronounced positive correlation between the activity of ovarian aromatase, calculated on day two of the menstrual cycle, and the intensity of aromatase fluorescence in cumulus cells (Spearman, r = 0.75, p = 0.001, n = 16). (For citation: Yakovlev PP, Krylova JuS, Mekina ID, et al. Ovarian aromatase activity: Evaluation methods and the clinical significance in IVF protocols. Journal of Obstetrics and Women’s Diseases. 2018;67(2):61-69. doi: 10.17816/JOWD67261-69).