Danazol, an isoxazol derivate of ethinyltestosterone, was used in the treatment of thirtyone postmenopausal patients with endometrial hyperplasia at the dose of 400 mg/day for three months. A profound growth-inhibitory effect of danazol on human endometrium was previously suggested. The present study shows that this compound is effective to reverse the endometrial hyperplasia to normal endometrium in 97% of cases. Atrophic changes of endometrial hyperplasia to normal endometrium after danazol-treatment were observed in 68% of patients. These results strongly suggest that danazol has a significant antiproliferative effect on the endometrium.