Eighteen women with a premature menopause underwent assessment of serial serum oestradiol (E2) and progesterone levels and endometrial histology and function. Patients received continuous transdermal E2, and progesterone either orally or vaginally for 14 days. Physiological levels of E2 were attained. Significantly higher levels of progesterone were achieved with vaginal progesterone (P less than 0.01). Endometrial biopsies obtained during E2 replacement demonstrated normal proliferative features. Expression of an oestrogen related antigen (ER-Ag) was localized in the cytoplasm of the epithelium. After 3 days of progesterone replacement the endometrium showed normal secretory features and expression of ER-Ag in the stroma as well as in the glands. After 7 days of progesterone supplementation, vaginal administration produced a more consistent physiological appearance than oral administration. Thus transdermal E2 combined with vaginal progesterone is a highly satisfactory combination for establishing a physiological endometrium in women with premature ovarian failure.