Gitelman's syndrome is a variant of Bartter's syndrome characterized by hypocalciuria and hypomagnesemia. The administration of thiazide diuretics may induce a subnormal increase of urinary Na+ and Cl- excretion in patients with Gitelman's syndrome, consistent with the hypothesis that less Na+ and Cl- than normal is reabsorbed by the thiazide-inhibitable transporter in Gitelman's syndrome. Specific mutations of NaCl cotransporter, coupled with mutant NaCl cotransporter expression studies clearly demonstrated that many of the characteristics of individuals with Gitelman's syndrome are explained by lack of function of NaCl cotransporter. We recently diagnosed a patient with Gitelman's syndrome by performing the thiazide and furosemide tests, and it is suggested that the clearance studies by diuretic administration may be of diagnostic help in Gitelman's syndrome.