A morphologic study was performed on the endometrium in 37 asymptomatic postmenopausal women under effects of cyclically administered oestrogens. Eighty-seven postmenopausal women were taken as control group. Medroxyprogesterone acetate (MPA), 10 mg daily, was administered in association with two types of oestrogen replacement therapy: conjugated equine oestrogens 0.625 mg (CEE) or transdermal 17 beta-oestradiol 0.05 mg (E2-TTS). Endometrial biopsies were taken under hysteroscopic control before treatment and on days 8-12 of combined therapy at the 6th month. Follow-up at 12 and 18 months was only performed in 8 and 5 patients, respectively, with transdermal 17 beta-oestradiol treatment. Various types of endometrial response were identified from atrophy to hyperplasia and secretory patterns. No atypical hyperplasia was found. All cases of simple or complex hyperplasia showed a regression after increased MPA dosage treatment (20 mg). This work is aimed at investigating the endometrial response during sequential cyclic therapy by using morphologic criteria based on hysteroscopy and histology. A large number of patients with hyperplasia can be detected with target biopsy under hysteroscopy, thus playing an important role in the management of patients during replacement therapy in research protocols.