The aim of the present study was to evaluate the effect of medical treatment of ovarian endometriomas on cyst diameter, associated pain, and ovarian reserve as measured with antral follicle count (AFC) and anti-Mullerian hormone (AMH). In this prospective study, 32 patients with unilateral endometrioma associated with pelvic pain, underwent 6-month medical treatment with dienogest. Before treatment, and at the end of 6 months of treatment, patients underwent evaluation of endometrioma diameter and AFC at transvaginal ultrasonography, measurement of AMH, and evaluation of associated pain. Mean cyst diameter was 4.0 ± 1.3 cm at baseline, and 2.4 ± 1.2 cm after 6 months of treatment (p < .0001), for a reduction in diameter of 40% and a reduction in volume of 79%. Mean visual analog scale score at enrollment was 6.3 ± 2.0, with a significant improvement at 6 months (0.9 ± 1.0, p < .0001). AFC for the affected ovary improved from 4.2 ± 2.8 at baseline, to 8.6 ± 4.2 cm after 6 months (+105%; p < .0001). AMH did not change significantly from baseline (3.40 ± 2.32 ng/mL) to end of treatment (2.80 ± 1.90 ng/mL, -18%, p = .27). Medical treatment with dienogest significantly reduces endometrioma diameter and associated pain, whereas the ovarian reserve appears to be preserved, with a significant improvement of AFC and no significant change in AMH.