Objective: Antidiuretic hormone (ADH) is involved in the response to stress and in depression and anxiety. However, studies on ADH in anorexia nervosa (AN) show conflicting results. A major reason for this may be methodological challenges due to short half-life of ADH in circulation and rapid degradation in vitro. To overcome these obstacles, copeptin, the C-terminal fragment stemming from the ADH precursor, has been increasingly used as a stable clinical measure for ADH. Furthermore, copeptin has been recognized as a biomarker of insulin resistance in obesity. Methods: We measured fasting copeptin in plasma from 25 normohydrated, stable women with AN (BMI 13.0 ± 2.0) and 25 age-matched women. Results: No difference in copeptin levels was found (6.8 ± 1.8 vs. 5.5 ± 0.5 pmol/L). Confirmatory, copeptin concentrations were correlated to insulin resistance assessed by the homeostasis model assessment of insulin resistance. Discussion: We report for the first time that copeptin level as a marker of ADH activity is not altered in fluid- and electrolyte-stabilized patients with severe AN patients, indicating that ADH may not be crucial in the pathophysiological involvement of psychologic stress in AN.