Twenty-three white women, ages 18 to 42, with normal menses prior to running were studied. Miles per week varied from 10 to 70 for a period of 1 to more than 10 years. Of these, 6 were amenorrheic (AM), 14 had regular cycles (REG), and 3 with regular cycles became amenorrheic during the course of this study. The incidence of amenorrhea was higher in those less than 30 years of age (66.6%) than in the older group (9.0%); in those who ran 40 miles/week or less (37.5%) than in those who ran more (26.6%); and in the nulliparous (46.6%) than in the parous runners (25.0%). The age of menarche was significantly higher in the AM (13.8 +/- 0.5 years) than in the REG (12.2 +/- 0.3 years). Blood samples were collected between 12 and 24 hours after the last run for hormonal and sex-hormone-binding globulin (SHBG) measurements. Plasma estradiol (E2), SHBG, and LH were significantly lower in the AM than in the REG group. Furthermore, E2, LH, and prolactin were significantly lower in the AM group than in the control group. These results suggest that the incidence of secondary amenorrhea is higher in younger, nulliparous female runners and may be related to delayed onset of menarche.