Reproductive system function in women cross-country runners. Med. Sci. Sports Exercise, Vol. 14, No. 4, pp. 263-269, 1982. The incidence and etiology of altered menstrual cycle function in women engaged in endurance athletic activities were investigated by studying endocrine, anthropometric, and training parameters in 41 cross-country runners. The prevalence of altered menstrual cycle patterns was significantly higher in the subjects than in college-aged; 49% reported normal cycles and 51% were either oligomenorrheic (46%) or amenorrheic (5%). No significant differences between those reporting normal menstrual cycling (N) and those reporting oligo/amenorrhea (O/A) were found in the following areas: number of miles run/week, number of years of training, age when training began, sum of skinfold thicknesses, somatotype, or post-exercise levels of growth hormone, prolactin, or hematocrit. However, a difference (P less than 0.05) was found in the mean age of menarche (N = 12.9 +/- 0.3 yr; O/A = 14.3 +/- 0.5 yr). In addition, more O/A (68%) than N (42%) began training in the year of or prior to menarche. Evaluation of seven runners from one school who qualified for the national meet (1 amenorrheic, 5 oligomenorrheic, and 1 normal) revealed that the basal estrogen, progesterone, prolactin, and thyroid hormone levels were normal and that there were normal luteinizing hormone and follicle-stimulating hormone responses to synthetic gonadotropin-releasing hormone (GnRH). These data are consistent with an alteration of hypothalamic-pituitary-ovarian function above the level of the pituitary.