Postmenopausal women received 1 mg estrone sulfate a day for 3 weeks in 12 sequences separated by a 7-day interval without medication and every second time by a 10-day treatment with natural progesterone, 100 mg 2 times daily (n = 16), or tamoxifen, 10 mg 2 times daily (n = 17). Both treatments alleviated climacteric symptoms, maturated the vaginal epithelium, and decreased follicle-stimulating hormone, luteinizing hormone, and prolactin concentrations while lipid metabolism remained unaltered. In the estrogen-progesterone group 4 women (25%) were persistently amenorrheic, 12 (75%) experienced 1–5 bleedings per year, and 4 women had proliferative endometrium after treatments. In the estrogen-tamoxifen group 11 women (65%) were amenorrheic, 6 (35%) had 1–4 bleedings per year, and 5 had proliferative endometrium. With the present treatment schedules, tamoxifen was more effective than natural progesterone in inhibiting estrogen stimulation of postmenopausal endometrium.