Polyestradiol phosphate is a polymeric ester of estradiol -17 beta and phosphoric acid. The large molecule has very weak estrogenic properties but is a strong inhibitor of several enzymes, e.g. acid and alkaline phosphatases and hyaluronidase. Given by injection it acts as a depot estrogen slowly releasing estradiol during several months. Given to male patients with prostatic carcinoma, it acts as weak inhibitor of gonadotrophins and testicular hormone production. For the treatment of prostatic carcinoma in 105 patients it gave good palliation and over-all survival 38% after five years. In a new series of 126 patients, polyestradiol phosphate was combined with ethinylestradiol. Over-all survival at five years in this group was 62%. It is suggested that polyestradiol phosphate is as effective as other estrogen therapy for prostatic carcinoma and that the combination with ethinylestradiol is superior to the single drug therapy.