Estriol was administered for a six-month period as estrogen replacement therapy to 52 symptomatic postmenopausal women. Assays of serum follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrone, and estradiol were performed before and during therapy. During this period of administration, vaginal cytology, cervical mucus, and endometrial studies were performed. Clinical effectiveness was directly related to dosage (2 to 8 mg/day). Estriol (8 mg/day) failed to induce endometrial proliferation and proved a poor suppressor of FSH and LH. This agent's capacity to relieve vasomotor instability and improve vaginal maturation without notable side effects is sufficient reason to include this drug in the management of the postmenopausal syndrome.