Dienogest (DNG), a progestin of 19-nortestosterone derivative, has good oral bioavailability and is highly selective for progesterone receptors. Owing to its antiovulatory, antiproliferative activities in endometrial cells, and its inhibitory effects on the secretion of cytokines, DNG is expected to be an effective treatment for endometriosis. Progesterone receptor-binding affinity is higher for DNG than for progesterone. Several pilot studies demonstrated that after 24 weeks of DNG treatment, there was a significant decrease in terms of dysmenorrhea, premenstrual pain, dyspareunia and diffuse pelvic pain. Most of the cases of genital bleeding occurring in the DNG treatment were spotting or breakthrough bleeding, which decreased with continued treatment and resolved either during treatment or after the end of treatment. The therapeutic effects of DNG 2 mg/day and norethisterone acetate 10 mg/day for endometriotic symptoms during a period of 24 weeks were almost similar. The only disadvantage of DNG seems to be the irregular bleeding. Good efficacy and tolerability of DNG in patients with endometriosis have been demonstrated in an open, randomized European clinical trial as compared with norethisterone acetate. In Japan, a Phase III, randomized, double-blind, multicenter, controlled trial was conducted to compare the efficacy and safety of DNG with intranasal buserelin acetate in patients with endometriosis. The study demonstrated that DNG is as effective as intranasal buserelin acetate in alleviating endometriosis symptoms, and causes less bone mineral density loss, resulting in the use on a commercial basis for endometriosis patients in Japan from 2008. This paper provides summarized data on this new promising drug for endometriosis.