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OBJECTIVE To evaluate the effects of estetrol 15 mg/drospirenone 3 mg on ovarian function. STUDY DESIGN Single-center, randomized, open-label, parallel study in healthy young women with proven ovulatory cycles. Participants received either estetrol 15 mg/drospirenone 3 mg (E4/DRSP) (n=41) or ethinylestradiol 20 µg/drospirenone 3 mg (EE/DRSP) (n=41) in a 24/4-day regimen for 3 consecutive cycles. Follicular size and endometrial thickness were measured by transvaginal ultrasound every three days in cycles 1 and 3. Blood was sampled for hormone analysis. Ovarian function expressed as Hoogland score was based on follicular size, serum estradiol (E2) and progesterone (P) concentrations. Ovulation was defined as a ruptured follicle-like structure >13 mm with serum E2 concentrations >100 pmol/L and serum P concentrations >5 nmol/L. We assessed return of ovulation after treatment cessation, and safety throughout the study. RESULTS None of the participants ovulated with E4/DRSP use, while one participant ovulated once and one participant ovulated twice during EE/DRSP treatment. Most participants had a Hoogland score of 1 (no ovarian activity) in cycle 1 (85.0% and 82.9% of participants on E4/DRSP and EE/DRSP, respectively) and in cycle 3 (65.8% and 83.8%, respectively). E4/DRSP suppressed follicle-stimulating hormone and luteinizing hormone to a lesser extent than E4/DRSP, whereas both treatments comparably suppressed E2 and P and endometrial thickness. Return of ovulation occurred, on average, 15.5 days after E4/DRSP treatment discontinuation. E4/DRSP was safe and well-tolerated. CONCLUSIONS E4 15 mg/ DRSP 3 mg results in adequate ovulation inhibition and ovarian function suppression, comparable to a marketed combined oral contraceptive containing EE/DRSP. IMPLICATIONS STATEMENT Treatment with E4 15 mg/DRSP 3 mg showed complete ovulation inhibition, despite less suppression of follicle-stimulating hormone and luteinizing hormone compared to EE/DRSP. If it becomes commercially available, E4/DRSP, containing a naturally occurring estrogen, should be as effective as EE/DRSP.