Objective. To estimate the level of ovarian reserve markers, including activin A, in unilateral ovarian endometriomas in relation to their size. Subject and methods. Examinations were made in 64 reproductive-aged infertile patients having ovarian endometriomas measuring ≤3 cm (study Group 1A, n=35) and more than 3 cm in diameter (study Group 1B, n=29); a control group consisted of 33 reproductively healthy patients of the same age. The basal serum levels of follicle-stimulating and luteinizing hormones, estradiol, anti-Müllerian hormone, inhibin B, and activin A were examined using an enzyme immunoassay and ultrasonography, by determining the number of antral follicles and resistivity index in the ovarian arteries. Biometric analysis was made using a Statistica-6 package and Microsoft Excel’s capabilities. Results. The levels of follicle-stimulating, luteinizing and anti-Müllerian hormones, inhibin B, and activin A did not vary with the size of an ovarian endometrioma in the study groups 1A and 1B and control group (p>0.05). There was a higher serum estradiol level in study group 1B (266.0 pmol/l) than that in the control group (136.0 pmol/l) (p<0.05). In study group 1B, the number of antral follicles (4 [0; 10]) decreased with a higher resistivity index (0.62±0.03) in the ipsilateral ovarian arteries as compared to the control group (10 [7; 10] and 0.58±0.01, respectively (р<0.05). Conclusion. The biochemical ovarian reserve markers, including activin A, did not vary with the size of an ovarian endometrioma as compared to those in the healthy women. There was a correlation between hyperestradiolemia and ovarian endometrioma sizes. As the endometrioma increased in size, the number of ovarian antral follicles decreased with a higher resistive index in the ipsilateral ovarian arteries as compared to those in the healthy women.