To evaluate the persistence of endometriosis and the morphologic changes occurring in endometriotic foci after hormonal therapy, microsurgical resection of ovarian endometriosis was performed in 148 women. After hormonal therapy (n = 116), a high prevalence of active endometriosis without signs of degeneration was found. Mitotic index was similar in the group of untreated women (n = 32) and in the three groups of women treated either with lynestrenol (Organon, Oss, The Netherlands), gestrinone (Roussel UCLAF, Paris, France), or buserelin (Hoechst, Frankfurt, West Germany). In conclusion, hormonal treatment leads to an incomplete suppression of endometriotic foci.