Multiple endocrine determinations were carried out on 101 patients with anorexia nervosa. Ninety-five percent of the patients studied were female, and in 94% of patients the anorexia nervosa began before 30 years of age. Evidence of gonadal dysfunction was the predominant manifestation, both clinically and by laboratory studies. Amenorrhea occurred before or concurrent with onset of weight loss in 65% of the women. The average weight loss was 28% of the weight before illness began. In an additional 11%, the disease began before menarche. The mean age of menarche in patients with secondary amenorrhea was 13 years. Urinary excretion of pituitary gonadotropin was undetectable in 44 of 65 patients and was below 19 rat units per 24 hours in the remaining patients. Serum luteinizing hormone level was below 8 microgram/dl in 15 of 27 patients studied and serum follicle-stimulating hormone was below 10 microgram/dl in 7 of 27 patients studied. Mean serum or urinary estrogens, or both, were low in more than 50% of the patients. Elevation of serum corticosteroids or loss or reversal of diurnal variation, or both, was noted in 50% of patients. Fasting serum growth hormone levels were elevated in 45% of the patients. Mean total and free serum thyroxine, thyroid-stimulating hormone, and triiodothyronine levels were low. These hormonal alterations in the hypothalamic-pituitary axis in patients with anorexia nervosa probably represent adaptive and protective mechanisms for chronic starvation and weight loss.