In eight women with ovarian failure, we induced histologically normal endometrial function during a preparatory cycle consisting of sequential administration of estrogen and progesterone. During a subsequent cycle, endometrial stimulation was synchronized with surrogate-embryo transfer performed on days 16 to 21. Among the eight women, two pregnancies were established by embryo transfer on days 18 and 19. In both women, ovarian tissue was absent, and these patients therefore serve as an in vivo model for the isolated effects of estrogen and progesterone on implantation and maintenance of pregnancy. Treatment with exogenous estrogen was mandatory up to the 11th week of gestation, and treatment with progesterone until the 18th to 22nd weeks. We conclude that it is biologically feasible to simulate the essential hormonal and endometrial milieu of a fertile menstrual cycle and early gestation solely by the administration of estrogen and progesterone. Days 18 to 19 of the cycle are recommended for successful embryo implantation with this treatment program. (N Engl J Med 1986; 314:806–11.), Young women who lack ovaries or whose ovaries are not functional make up a relatively small, though important, group of sterile patients. In women with surgical castration or gonadal dysgenesis, the ovaries are absent or are replaced by a streak of fibrous tissue. Gonadal dysgenesis may present in a pure form with a 46,XX chromosomal complement or in conjunction with Turner's syndrome (45,XO). Patients with gonadal dysgenesis have primary amenorrhea and sexual infantilism. Patients with premature ovarian failure (premature menopause) usually present with permanent ovarian failure occurring after menarche but before the normal menopausal age. Premature ovarian failure has been.