We present a case of Gitelman's Syndrome in a 20 year-old woman who came to our service with weakness, asthenia, leg cramps and tetany. Laboratory studies revealed metabolic alkalosis with hypokalemia, hypomagnesemia and low calcium in a 24-hour urine test. The diagnosis of this syndrome is made in some cases during adult life because this syndrome is asymptomatic over several years. Gitelman's Syndrome is autosomal recessive as is Bartter's Syndrome. The gene is located in chromosome 16q, which encodes the cotransporter Na/Cl sensitive to thiazide in the distal convoluted tubule. The defect of cotransporter produces an alteration of sodium reabsorption that causes electrolytic disorders typical of this Syndrome and different from Bartter's Syndrome. The typical electrolytic alterations are hypocalciuria and hypomagnesemia secondary to high urinary magnesium excretion. The prognosis of this syndrome is excellent and treatment consists in correction of serum electrolytes with oral administration of magnesium and potassium. In spite of this treatment, in some cases it is very difficult to reach normal serum levels of magnesium because of the high doses of oral magnesium, which produce common crises of diarrhea that increase magnesium gastrointestinal losses.