In prospective studies, we have determined the endometrial histologic characteristics and patterns of vaginal bleeding in 12 perimenopausal or postmenopausal women during administration of transdermal estradiol, 0.05 mg daily, given either alone or in combination with a progestogen. In the first study, we administered transdermal estradiol in cyclical fashion for 3 months. Outpatient curettage at pretreatment produced no endometrial sample or tissue too scant for assessment from 10 of the 12 patients (83%). At the end of therapy, proliferative or nonsecretory endometrium was diagnosed in nine patients (75%). Eight patients experienced treatment-related vaginal bleeding but no regular pattern, and seven patients reported breakthrough bleeding. Eight patients participated in the second study in which transdermal estradiol was administered continuously and norethindrone, 0.35 mg, was added for 12 days of each calendar month. A further curettage was performed at the end of treatment, and proliferative endometrium was the most common finding. No endometrial hyperplasia was observed. Only one patient experienced breakthrough bleeding. There were no consistent changes with time in the number of patients bleeding each month or in the duration or heaviness of the bleeding.