There are many cases of chronic dehydration in babyhood. In adults, however, there are few cases other than those caused by disorders of the thirst center or endocrinological diseases. We treated a patient with a rare case of gastrointestinal neurosis with hypernatremic hyperchloremia caused by chronic dehydration. This 22-year-old male university student visited my clinic complaining of weight loss, loss of appetite (restrictions in dietary intake), sense of abdominal fullness, constipation and general fatigue. He had undergone medical treatment for chronic gastritis for two years. He weighted 40.0kg and was 173.0cm tall. Laboratory findings at his first visit showed serum natrium at 164mEq/l and serum chlorine at 122mEq/l. His weight loss and abnormal levels of serum electrolytes were due to his intentional ristriction of fat and water intake in his diet. These results were explained in detail to make him understand his errors in taking water and food. As he was prepossessed with the symptoms of stomach and intestines and tended to have a nervous temperament (Morita shinkeishitsu), he underwent modified Morita therapy as an outpatient. As he became able to take water and fat freely, serum electrolyte levels normalized and his symptoms disappeared gradually. He gained 16kg in six months. This case shows that in treating chronic diseases it is important not only to prescribe medication and general dietary instructions, but also to carefully monitor the patient's lifestyle, eating behaviors and personality.