The morphological effect of a high-dose progestogen (Depo-Provera) on highly differentiated endometrial carcinoma was studied. Thirteen patients were treated by progestogen only, given at the beginning ofthe treatment in a weekly dose of 1,000 mg for 3 month. Then, progestogen 500 mg was administered for 12 months. Nine patients were operated after the treatment; in 7 of them, a total histological regression was observed. Histological tests of the intraoperatively gained preparations showed ‘disappearance’ of the original carcinomatous process; only a serious atrophy of the endometrium could be detected.