To determine whether a significant energy drain during adolescence had a significant effect on puberty and normal reproductive function, 15 ballet dancers, aged 13--15 yr, who maintained a high level of physical activity from early adolescence were followed for 4.0 yr. Menarche was remarkably delayed in this group, occurring at a mean of 15.4 yr, significantly different (P < 0.01) from normal controls (12.5 yr) and normal music students (12.6 yr). In 2 dancers aged 18 yr, primary amenorrhea has persisted. While premenarchial, all of the dancers had varying breast development (Tanner stages 2--4) and low to low normal gonadotropin levels, normal PRL and T4 levels, and normal skull x-rays. The dancers' mean body weight and calculated body fat were significantly less than in controls (P < 0.05). The progression of sexual development and the onset of menarche correlated in 10 or 15 subjects with a decrease in exercise and/or injury causing forced rest of at least 2-month duration. During this interval, weight gain was minimal or absent, with no significant change in body composition. A significant dichotomy in the order of pubertal development was also noted; while breast development and menarche were delayed, pubic hair development was not affected. Reversion to the amenorrheic state occurred in 11 of 13 patients with a return to exercise without a change in weight. In conclusion, energy drain may have an important modulatory effect on the hypothalamic pituitary set point at puberty and, in combination with low body weight, may prolong the prepubertal state and induce amenorrhea.