Estradiol phosphate, or estradiol 17ò-phosphate, also known as estra-1,3,5(10)-triene-3,17ò-diol 17ò-(dihydrogen phosphate), is an estrogen which was never marketed. It is an estrogen ester, specifically an ester of estradiol with phosphoric acid, and acts as a prodrug of estradiol in the body. It is rapidly cleaved by phosphatase enzymes into estradiol upon administration. Estradiol phosphate is contained within the chemical structures of two other estradiol esters, polyestradiol phosphate (a polymer of estradiol phosphate) and estramustine phosphate (estradiol 3-normustine 17ò-phosphate), both of which have been marketed for the treatment of prostate cancer.