The development of the reproductive axis is thought to be a gradual process, but our understanding of the complex endocrine changes that accompany the transition from premenarche to reproductive life in women has been hampered by the paucity of longitudinal studies. We studied 112 premenarchal Caucasian females at 6-month intervals over 4 yr and obtained a detailed reproductive and dietary history. We quantified reproductive hormones in 24-h urine collections as a measure of daily output and measured body composition biometrically and with the use of dual energy x-ray absortiometry scans. The percent body fat did not change appreciably in the study period (range, 21-24%) and was unrelated to menarche. Sex steroid and gonadotropin levels changed exponentially in the year approaching menarche. FSH levels peaked at menarche and then progressively declined thereafter. Estradiol output increased rapidly in the year approaching menarche and then plateaued thereafter. The frequency of menstrual bleeding increased rapidly and plateaued at 1 yr postmenarche. At 1 yr, 65% of these adolescent women had established a pattern of 10 or more menstrual episodes/yr, and by 3 yr postmenarche this figure exceeded 90%. There were no significant changes in dietary intake of protein, carbohydrate, or fat in the same period. Menarche occurs as a result of rapid maturation of the reproductive axis and heralds the reestablishment of a negative sex steroid feedback loop that parallels the adult threshold. These events appear to develop independent of changes in body composition and diet, but may reflect the improved nutrition and socioeconomic status of the late 20th century.