Since women have become more involved in physical fitness and competitive endurance sports, the incidence of menstrual dysfunction has increased. Long-distance running and other sports may lead to alterations in gonadotropins, androgens, estrogens, progesterone, or prolactin, which in some women may directly or indirectly result in amenorrhea or infertility. The effects of running and strenuous exercise on the menstrual cycle and reproductive hormones remain controversial. Reported incidences of menstrual dysfunction vary widely, and many factors have been implicated in the onset of this problem. Exercise associated menstrual dysfunction seems to occur more frequently in nulliparous athletes, in athletes with delayed menarche, and in athletes with low body fat. It is important to realize that disruption of the menstrual cycle, ranging from mild changes in flow to amenorrhea, is a relatively common problem for the female athlete engaged in strenuous endurance sports. Yet no evidence exists at present to indicate conclusively that this menstrual dysfunction is harmful to the female athlete's reproductive system.