Women with moderate weight loss (10-15% of ideal weight), as well as women with the severe weight loss of anorexia nervosa (30% of ideal weight), have secondary or primary amenorrhoea. A high proportion of well-trained dancers and athletes also have amenorrhoea, though weight may be in the normal range, since muscles are heavy (80% water, compared to 5-10% water in adipose tissue). The amenorrhoea is usually reversible with weight gain, decreased exercise or both. The amenorrhoea is due to hypothalamic dysfunction; the pituitary-ovary axis is intact, suggesting that this type of amenorrhoea is adaptive, preventing an unsuccessful pregnancy outcome. Evidence is presented that the high percentage of body fat (26-28%) in mature women is necessary for regular ovulatory cycles. Target weights for height are given for the evaluation and treatment of primary and secondary amenorrhoea due to weight loss. The high percentage of body fat in women may influence reproductive ability directly: (1) as an extragonadal source of oestrogen by aromatization of androgen to oestrogen; (2) by influencing the direction of oestrogen metabolism to more potent or less potent forms; or (3) by changes in the binding properties of sex-hormone-binding globulin. Indirect signals may be of abnormal control of temperature and changes in energy metabolism, which accompany excessive leanness.