This chapter will focus on the role of PRMs in the treatment of endometriosis. Efficacy has been demonstrated in pre-menopausal women with symptomatic endometriosis treated for 6 months with 12.5, 25 or 50 mg of CDB-4124. The most favorable response in this small study occurred with the highest dose (50 mg). This small proof of concept and safety study is very important in furthering the understanding of the effect of PRMs in general and CDB-4124 in particular in the treatment of endometriosis as well as the effect on bleeding patterns and the endometrium. At the present time, this intermittent regimen appears to be the most appropriate for long-term treatment. Future development of PRMs for the treatment of endometriosis may be the advent of compounds which are selective to the A or B subunit of the PR.