A 19-year-old girl, with the 45,XO non-mosaic Turner syndrome, presented having spontaneous regular menstrual activity. Hormonal analysis showed normal ovulatory cycles; histology of the ovaries demonstrated primary follicles. Nevertheless, the karyotype of the ovary and of the peripheral lymphocytes was pure 45,XO. Comment on ovarian function in the 45,XO karyotype is given.