Abnormal responses of serum prolactin (PRL) to luteinizing hormone-releasing hormone (LHRH) stimulation have been observed in anovulatory women and in hypogonadal patients. Various endocrinological abnormalities have been demonstrated in patients with anorexia nervosa (AN). The present study was undertaken to further investigate responses of serum PRL, growth hormone (GH), luteinizing hormone (LH) and follicle stimulating hormone (FSH) to LHRH stimulation in 65 patients with AN and in 12 patients with bulimia before therapy and in the AN patients after several months of treatment, and in comparison to 12 normal women of the same age. Serum PRL responses to LHRH were positive (peak PRL levels greater than 25 ng/ml and delta increase in PRL greater than 10 ng/ml) in 16.9% of AN and 16.6% of bulimic patients; they were negative (absent) in all controls. Following restoration of the AN patients to normal body weight, the PRL responses to LHRH became normalized in those patients whose eating disorder behavior also returned to normal. However, in those patients whose eating disorder patterns continued to be abnormal, abnormal PRL responses persisted. The bulimic patients were of normal body weight, and yet had abnormal PRL responses. Thus, the responses of PRL correlated more closely with the behavior of the underlying eating disorder rather than with body weight gain or normal body weight.