Background Adolescents with eating disorders (EDs) may present not only with abnormal eating behaviors but also with abnormal drinking behaviors varying widely. These behaviors include water loading to cheat on weight measurements, to feel full and suppress appetite and/or to induce vomiting; as well as restricting fluid intake in addition to food. Method We present a 16-year-old female adolescent with anorexia nervosa restrictive type and major depressive disorder who was hospitalized due to acute food refusal and developed generalized seizures due to dilutional hyponatremia in consequence of consuming excessive amount of water. Psychiatric diagnoses were made according to 'The Diagnostic and Statistical Manual of Mental Disorders' (5th ed.; DSM-5) criteria. Results After starting nutritional rehabilitation with a low calorie meal plan to avoid refeeding syndrome, a weight gain of 2 kg was noted in the second day of hospitalization. At the bedside visit, she was observed in a disoriented manner and consecutively in seconds, lost consciousness with a generalized tonic-clonic seizure lasting 2 min. Her serum sodium level was measured as 116 mEq/L, which was normal at the time of admission. It was later learned that she secretly ingested 19 L of water in a short amount of time. She regained consciousness and no further seizures were observed after intravenous sodium deficit correction and fluid restriction therapy. Her serum sodium level was normalized (137 mEq/L) within 12 h. Conclusion A thorough clinical assessment of hydration and drinking behaviors as well as eating behaviors is essential for patients with EDs to avoid serious medical complications with high mortality and morbidity during follow-up. It is interesting that this amount of fluid consumption in such a short period of time did not present to the clinic with vomiting, gastric dilatation or bowel irrigation symptoms in a case with acute food refusal and restriction for a year, instead absorbed very quickly causing acute and severe symptomatic hyponatremia with generalized seizures.