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Psychogenic polydipsia (PPD), a clinical disorder characterized by polyuria and polydipsia, is a common occurrence in patients with psychiatric disorders. Hyponatremia (as a result of polydipsia) can progress to water intoxication that is characterized by confusion, lethargy, psychosis, and seizures. There are a number of studies shows that uncomplicated polydipsia and compulsive water consumption are frequently seen in anorexia nervosa (AN), also hyponatremia is infrequently reported in the constellation of metabolic abnormalities in patients with eating disorders. In this case report, we will discuss an adolescent with AN, who had water intoxication after excessive water consumption for the use of weight loss. A 15 years old girl was referred because of excessive water consumption, polyuria and food restriction. Her complaints started after she decided to set dieting as she thought she had put on too much. After reading information about dieting by drinking water, she had increased water consumption. Especially at the evening hours she was drinking 10-12 liters of water and that has continued for 3 months. In 3 months she has lost weight to 42 kg from 55 kg. One month before she was referred to Child and Adolescent Psychiatry Clinic, after she had been referred to an emergency service due to symptoms of dizziness, headache and syncope after consuming 10 liters of şuid in 6 hours. She was hospitalized in an intensive care unit with prediagnosis of PPD, hyponatremia and diabetes insipidus as her sodium level was 116 mmol/L (normal limits: 135–145 mmol/L). Fluid consumption was monitored throughout hospitalization and after normalization of her serum sodium level, she had referred to the child and adolescent psychiatry clinic as her clinical, biochemical and radiological examinations were all in normal levels. She was eating beggarly and only once a day and drinking at least 2 liters of water at the evening hours. The patient was prediagnosed as anorexia nervosa and PPD, and had decided to be followed as outpatient. Some behavioral targets were determined like decreasing the amount of şuid intake. Fluoxetine treatment was started. She has been followed up for a year at monthly intervals and in due course dietary harmonization and amount of şuid intake returned to normal. It has been stated that the main reasons of polydipsia in patients with AN, are appeasement of hunger, detoxifying, decreasing the amount of calorie intake and hiding the body weight. PPD is a situation that is not rare in AN but rarely causes clinical complications; seizures and death due to cerebral edema are reported. In conclusion, patients with eating disorders may differ in şuid consumption quantitavely. Early recognition of polydipsia in these patients is important because of its lethal complications. Therefore observation of the amount of şuid consumption should be an integral part of clinical examination in patients with eating disorders. There is a need for further studies about the triggering environmental, biological and genetic factors of polydipsia in eating disorders.