The pharmaceutical preparation for treating endometriosis contains at least 28, preferably 30, daily dose units, each of which contain dienogest, cyproterone acetate, or chlormadinone acetate at a daily dose that is at most twice that required to inhibit ovulation together with one or more pharmaceutical aids and/or carriers. The daily dose units are administered in a method of prophylaxis and/or therapy of endometriosis continuously during a time interval of at least 169 days or 25 weeks, preferably more than two years. The method effectively reduces endometriosis and associated pain, while undesirable side effects including bone density decrease are reduced or eliminated.