The average age for the onset of menopause in humans is about 51 years. On the other hand, premature ovarian failure (POF) is generally defined as the onset of menopause before the age of 40 years. Telomeres have been extensively examined as a mitotic clock. Telomeric DNA is elongated by telomerase. We analyzed the telomerase activity of 20 patients with normal ovaries and 5 patients with POF. Telomerase activity was present in the normal ovaries, however it decreased with age. Eight normal ovaries under 38 years of age showed significantly higher telomerase activity among the women with a regular menstrual cycle. Two POF patients showed high telomerase activity and 3 showed low telomerase activity. Our findings indicate that telomerase is present in the normal human ovary and that telomerase activity decreases with age. Patients with follicle dysfunction showed high telomerase activity and those with follicle depletion showed very low telomerase activity. Based on these results, we speculated that the decline of telomerase activity in the ovary is related to primordial follicle depletion with age and telomerase activity can be used as a marker of the ovarian functional age.