Vilaprisan, a small molecule progesterone receptor antagonist is being developed by Bayer HealthCare Pharmaceuticals (formerly Bayer Schering Pharma) for the treatment of endometriosis and uterine leiomyoma. Hormonal imbalance observed in women with endometriosis is a potential target for treating endometriosis. Vilaprisan is a highly selective steroidal progesterone receptor modulator (SPRM). It is a partial agonist of progesterone receptor, which means that the drug activates progesterone receptors to a certain degree upon binding. This triggers a cascade of biochemical reactions that result in the suppression of prostaglandin production. This, in turn, relieves symptoms such as pain and bleeding. Modulating progesterone by taking vilaprisan might help in treating endometriosis over the long term. Phase I and II studies give encouraging results on the efficacy of vilaprisan at different doses. Like other SPRMs, vilaprisan induces benign changes of endometrium (PR modulator-associated endometrial changes, PAECs). These disappear as treatment is discontinued. Unlike GnRHa treatment, neither UPA nor vilaprisan induce hypoestrogenism and associated symptoms. Phase III studies are ongoing to confirm efficacy and safety of vilaprisan in long-term treatment of symptomatic fibroids.