Objective: To investigate whether serum concentrations of antimullerian hormone may be used as a marker for ovarian aging. Design: Longitudinal observational study. Setting: Academic research center. Patient(s): Forty-one normo-ovulatory premenopausal women and 13 healthy postmenopausal women. Main Outcome Measure(s): Concentrations of serum antimullerian hormone (assessed on two occasions 2.6 +/- 1.7 years apart), FSH, inhibin B, and estradiol and number of ovarian follicles on ultrasonography. Result(s): Concentrations of antimullerian hormone decreased significantly over time (median value, 2.1 mug/L [range, 0.1-7.4 mug/L] at visit 1 vs. 1.3 mug/L [range, 0.0-5.0 mug/L] at visit 2), whereas the number of antral follicles and levels of FSH and inhibin B did not change. During visits 1 and 2, concentrations of antimullerian hormone correlated with age (r = -.40, P=.01 and r = -.57, P<.001, respectively); number of antral follicles (r = .66, P<.001 and r = .71, P<.001); and, to a lesser extent, with FSH level (r = -.29, P=.07 and r = -.37, P<.05) but not with inhibin B levels. Conclusion(s): Serum concentrations of antimullerian hormone decreased over time in young normo-ovulatory women, whereas other markers associated with ovarian aging did not change. Concentrations of antimullerian hormone correlate with the number of antral follicles and age and less strongly with FSH level. Concentrations of antimullerian hormone may be a novel marker for ovarian aging. (Fertil Steril(R) 2002;77: 357-62. (C)2002 by American Society for Reproductive Medicine.).