We report the case of an asymptomatic patient presenting a severe chronic renal hypokalaemia. Once being sure of no diuretics use, two hypothesis can be mentioned for a normotensive patient presenting an hypokalaemia associated with a metabolic alcalosis: Bartter syndrome or Gitelman syndrome. The highlighting of low magnesaemia and hypocalciuria strongly concentrates the diagnosis on Gitelman syndrome. First, this has been strengthened by the results of renal function tests and later it has confirmed by molecular diagnosis with the identification of a known homozygous mutation on SLC12A3 gene. In the patient family, the same chromosomal abnormality has been found in the young sister. For these two patients the treatment ordered is an antikaliuretic diuretic, magnesium and potassium supplements. This case shows the difficulty to diagnose Gitelman syndrome: it is frequently mistaken for Bartter syndrome. The main differences between these two syndromes are magnesaemia and calciuria. Furthemore , patients with Gitelman syndrome are often asymptomatic, this explains why prevalence of this illness is probably underestimated.