Impaired growth in children without constitutional growth delay or known chronical disease is often linked to endocrine or gastrointestinal disorders. A rare cause is primary renal tubular dysfunction. A brother and a sister who were diagnosed as having Barter's syndrome, later corrected to Gitelman's syndrome, are presented. Both of these syndromes include hypokalemia, hypochloremia, metabolic alkalosis, usually hypomagnesemia, increased activation of the renin-angiotensin-aldosterone system, normal blood pressure and impaired growth. Patients with the classical Bartter's syndrome are treated with potassium chloride and a prostaglandin synthesis inhibitor, and patients with Gitelman's syndrome with magnesium chloride supplementation. We emphasize that measurement of serum electrolytes should be included in the investigation of children with impaired growth.