On September 27-29, 1993, the Study Group on Anorexia Nervosa (AN): Basic Mechanisms, Clinical Approaches and Treatment met in Geneva, Switzerland to discuss recent progress in research on anorexia nervosa, and to identify directions for future studies. Anorexia nervosa is a disorder of unknown etiology, without a specific curative treatment, affecting mostly individuals in adolescence and early adulthood, with significant morbidity and mortality, and having a major impact on psychosocial and vocational development. In anorexia nervosa there are severe disturbances in virtually every endocrine system, such as the hypothalamic-pituitary-adrenal (HPA) axis, the hypothalamic-pituitary-gonadal (HPG) axis, the hypothalamic-pituitary-thyroid (HPT) axis, the growth hormone (GH)/somatomedin C (IGF-1) system, and the central and peripheral arginine vasopressin (AVP) systems. Furthermore, classical neurotransmitter systems, such as the cholinergic noradrenergic, and serotonergic systems, are abnormally regulated in anorexia nervosa. New research data is also emerging on the abnormal regulation of immune function in this disorder. The Study Group concluded that even though several biological systems are abnormally regulated in anorexia nervosa, there is no biological test which is specific enough to make the diagnosis of the disorder. New directions for research in anorexia nervosa are identified and discussed in this report. Finally, the Study Group proposed future meetings to bring together clinical and pre-clinical (pharmacological, biochemical, and molecular) scientists studying topics, such as neuroendocrine function, which are important in the biology of anorexia nervosa.