This case report describes the management of a 49-year-old female with restricting-type anorexia nervosa and excessive compulsive exercising associated with rhabdomyolysis, high levels of serum creatine kinase (CK) (3,238 U/L), and marked hyponatremia (Na+: 123 mEq/L) in the absence of purging behaviours or psychogenic polydipsia; it is the first case report to describe exercise-associated hyponatremia in a patient with anorexia nervosa. The patient, who presented with a body mass index (BMI) of 13.4 kg/m2, was successfully treated by means of an adapted inpatient version of an enhanced form of cognitive behavioural therapy (CBT-E). Within a few days, careful water restriction, solute refeeding, and the specific cognitive behavioural strategies and procedures used to address the patient's excessive compulsive exercising and undereating produced a marked reduction in CK levels, which normalised within one week. Exercise-associated hyponatremia also gradually improved, with serum sodium levels returning to normal within two weeks. The patient thereby avoided severe complications such as cerebral or pulmonary oedema or acute renal failure and was discharged after 20 weeks of treatment with a BMI of 19.0 kg/m2 and improved eating disorder psychopathology.