Gitelman syndrome is an inherited renal tubular disorder characterized by hypokalemic metabolic alkalosis. It is distinguished from other hypokalemic tubulopathies, such as Bartter syndrome, by the presence of both hypomagnesemia and hypocalciuria. We report a case of Gitelman syndrome in a 10-year-old girl who presented for examination of persistent unexplained hypokalemia. She had no severe clinical symptoms but she had typical laboratory findings including hypokalemia, hypomagnesemia and normocalcemic hypocalciuria. Molecular analysis revealed a mutation in the exon 21 of the SLC12A3 gene which encodes the thiazide-sensitive sodium-chloride co-transporter expressed in the distal convoluted tubule (a guanine to adenosine substitution at nucleotide 2538). She was treated with oral potassium and magnesium supplements. This is the first report of genetically established diagnosis in Greece. Gitelman syndrome should be considered as a cause of persistent hypokalemia and genetic analysis might be a useful tool to confirm the diagnosis.