Abstract Study question Does intraovarian injection of autologous PRP affect ovarian function in women with POI? Summary answer Ovaries that were injected with PRP demonstrated a significant increase in follicles >4mm in size compared to the untreated contralateral ovaries. What is known already The injection of PRP into ovaries has been reported to improve subsequent ovarian response to ovulation induction in women with POI and is now widely used in fertility centers in women with POI as well as in women with premature ovarian aging (POA). Prospectively randomized studies are, however, lacking. Study design, size, duration This is a Randomized Controlled Trial with recruitment from July 2018 - November 2023. 40 patients were screened; 34 qualified. POI was defined by amenorrhea among women < 40 years old, with FSH > 30 mIU/mL, undetectable AMH, and no follicles >4mm. For each participant one ovary was randomly selected for treatment ( with the Sealed Envelope Ltd 2022 tool). The study was powered to detect a 20% response in the treated versus untreated ovary. Participants/materials, setting, methods For each patient (n = 34), 10mL of blood was drawn into ReganLab vacutainer and centrifuged. Platelets were re-suspended in 1.5mL of plasma. PRP was injected into the stroma of one randomly selected ovary under iv sedation and ultrasound control. Patients were followed with ultrasounds, estradiol, and FSH q3-4 days for two weeks. Our primary outcome was the development of new follicles >4mm in the treated vs. untreated ovary, using Chi-Square. Main results and the role of chance Study population description: age 33.9±5.5 years (range 21-40.4 years); BMI 23.4±4.2; FSH 96.7±27.6 mIU/mL; AMH 0.019±0.03 ng/mL; length of amenorrhea 2.2 years (3 months-14 years). 10 patients developed no follicles > 4 mm, 21/34 (62%) did develop follicles > 4 mm in treated ovaries; 9/34 (26%) developed a follicle > 4 mm in untreated ovaries (Chi-square = 8.59; P = 0.003). Most follicles developed without significant estradiol production; nine showed sufficient follicle development to initiate ovulation induction with initiation of gonadotropins, four produced eggs and one produced an embryo; one patient had four oocytes banked and resumed menses following PRP treatment for several months. Limitations, reasons for caution N/A Wider implications of the findings This demonstrates the first objective evidence that intraovarian PRP injections into POI ovaries initiate some follicle activation. Though the observed degree of activation was small, findings suggest that remaining follicles in women with POI can be activated through injections into ovaries, suggesting that better agents than PRP may be discovered. Trial registration number N/A