Many well-trained athletes, ballet dancers and women who diet excessively have secondary or primary amenorrhoea. Less extensive training or weight loss may result in anovulatory menstrual cycles, or a shortened luteal phase. These disruptions of reproductive ability are due to hypothalamic dysfunction, which is correlated with weight loss or excessive leanness. It is proposed that these associations are causal and that the high percentage of body fat (26-28%) in the mature human female may influence reproduction directly. Four mechanisms are known: (i) adipose tissue converts androgens to oestrogen by aromatization. Body fat is thus a significant extragonadal source of oestrogen; (ii) body weight, hence fatness, influences the direction of oestrogen metabolism to more potent or less potent forms; leaner women make more catechol oestrogens, the less potent form; (iii) obese women and young, fat girls have a diminished capacity for oestrogen to bind sex-hormone-binding-globulin; (iv) adipose tissue can store steroid hormones. An indirect mechanism may be signals of abnormal control of temperature and changes in energy metabolism, which accompany excessive leanness. The hypothalamic reproductive dysfunction results in abnormal gonadotrophin secretion: there is an age inappropriate secretory pattern of luteinizing hormone (LH) and follicle stimulating hormone (FSH), resembling that of prepubertal children. The secretion of LH and the responses to LHRH are reduced in direct correlation with the amount of weight loss. Other evidence from non-athletic and athletic women and mammals is presented in support of the hypothesis that a particular, minimum ratio of fat to lean mass is normally necessary for menarche (approximately 17% fat/body wt) and the maintenance of female reproductive ability (approximately 22% fat/body wt). Nomograms are given for the prediction of these critical weights for height from a fatness index; these weights are useful clinically in the evaluation of nutritional amenorrhoea and the restoration of fertility in underweight women. Evidence is presented that undernutrition and hard physical work can affect the natural fertility of populations, by the delay of menarche, a longer period of adolescent subfecundity, a longer birth interval and an earlier age of menopause. Data from a study of the long-term reproductive health of 2622 former college athletes compared with 2766 non-athletes show that the former college athletes had a significantly lower lifetime occurrence of breast cancer and cancers of the reproductive system, and a lower lifetime occurrence of benign tumours of these tissues, compared with the non-athletes.(ABSTRACT TRUNCATED AT 400 WORDS)