The synthetic steroid danazol has rapidly become the drug most commonly used in the hormonal treatment of endometriosis and its clinical and endocrine properties in women have been investigated extensively. However, little is known about the early effects of danazol on the endometrium. This study was performed in order to discover how soon after institution of danazol therapy atrophic changes can be observed in the endometrium. Ten patients with fibromyoma of the uterus and menorrhagia were treated with danazol at a dosage of 600 mg a day. During treatment, plasma estradiol values corresponded to those of the early follicular phase of a normal menstrual cycle. In 5 of the patients, endometrial biopsies were obtained after 8 weeks of therapy. Four of them had a thin endometrium of atrophic histological appearance. In view of these findings, biopsies were taken from a further 5 patients after only 4 weeks' treatment. All of these biopsies revealed a reduced endometrium and 3 of the 5 endometria were histologically clearly atrophic. Even if total resorption of endometriotic tissue usually takes a considerable time, the finding of atrophy of the uterine endometrium induced by danazol after only 4 weeks of treatment is consistent with the rapid improvement in clinical symptoms in patients with endometriosis.