The characteristics of 26 patients with presumptive premature ovarian failure have been examined. The initial diagnosis was based on any single serum follicle-stimulating hormone (FSH) concentration of greater than 40 mIU/ml in karyotypically normal women under 35 years of age with irregular menses or amenorrhea. Clinical manifestations were heterogeneous: some failed to undergo pubertal maturation, and other developed hypergonadotropic amenorrhea following several years of regular menses. Almost 70% experienced hot flashes. Three had thyroiditis. Nine of 18 patients had hormonal evidence of functioning ovarian follicles, and 4 of 9 women had viable oocytes on biopsy. Evidence of ovulation was noted in five patients, and spontaneous pregnancy occurred in one. These data emphasize the fallacy of using elevated FSH levels to diagnose irreversible ovarian failure and indicate the possibility of ovulation and pregnancy in some affected individuals.