The problem of menstrual dysfunction in women who engage in endurance training for participation in distance running events has been studied. Through survey, selected aspects of the personal, training, menstrual, and contraceptive histories of 168 women who were defined as runners, joggers, or controls were evaluated. In addition, defined subsets of the study subjects were evaluated for serum levels of pituitary and ovarian hormones and determination of percentage body fat. The data show significant differences among the 3 groups. It is concluded that menstrual dysfunction in distance runners is a real phenomenon. Presumably this is related to decreased percentage of body fat and/or minimal ovarian function secondary to diminished hypothalamic or pituitary hormone secretion.