Hypokalaemia is found in up to 20% of eating disorders, it can cause life-threatening conditions, such as cardiac arrhythmias, myopathy, rhabdomyolysis, nephropathy and paralysis. It often remains a diagnostic challenge, especially in young women without hypertension. Hypokalaemia in eating disorders usually develops as a result of diuretic improper use or gastrointestinal losses. After the exclusion of the most obvious causes, a concealed diuretic abuse associated with or without surreptitious vomiting and laxative abuse should be suspected, especially in young women concerned with their body image. A conclusive diagnosis may be difficult as such patients often vigorously deny diuretic intake. Anyway only a minority of patients with eating disorders (approximately 6%) abuse diuretics. This report describes a teen girl with a severe HK. A thorough medical history and proper tests allowed an early diagnosis and accurate treatment.