Progestin preparations are made of synthetic progesterone and have often been used for hormone therapy in gynecological patients with endometriosis or endometrial cancer. Hormone therapy using progestin is considered to be one of the effective means of treatment particularly when dealing with endometrial cancer (an estrogen-dependent tumor). Numerous reports have been published concerning its efficacy in advanced or recurrent cases of atypical endometrial hyperplasia or endometrial cancer. Dienogest has been developed as a fourth-generation progestin for hormone therapy for endometriosis that can be used with high safety for long periods of time. In Japan, dienogest has been recommended as a first-line drug for endometriosis-associated pain. However, its antitumor activity has also been attracting close attention following a report that this drug suppressed the proliferation in vitro of endometrial cancer-derived cell lines which failed to respond to other progestins such as medroxyprogesterine acetate (MPA). The mechanism for antitumor activity of dienogest is considered to differ from the mechanism for antitumor activity of conventional progestin preparations used for treatment of endometrial cancer. This drug is expected to be clinically applicable as a new drug for the treatment of endometrial cancer.