Fifteen women with anorexia nervosa were studied before and after weight gain. Basal plasma thyroid stimulating hormone (TSH) and prolactin (PRL), and the responses of both these hormones to thyrotropin releasing hormone (TRH), were normal. Basal plasma luteinizing hormone (LH) and follicle stimulating hormone (FSH) were low in patients who were emaciated, and their responses to gonadotropin releasing hormone (GnRH) were impaired. Both basal and stimulated levels of LH and FSH rose with weight gain, with a linear correlation between gonadotropin levels and body weight expressed as a percentage of standard. The FSH response became greater than normal in patients who had regained weight to more than 70% of standard, while the LH response to GnRH was exaggerated in those who had regained weight to more than 80%. Basal plasma estradiol (E2) levels were low at first, but returned to within the normal range in patients over 80% of standard. Menstruation resumed in some patients after they had regained weight. The relationship between body weight and gonadotropin levels appears to be an important feature of the menstrual disturbance in anorexia nervosa. The restoration of a normal body weight is a prerequisite for the resumption of menstruation in this condition, but other as yet unidentified factors may also be involved.