Gitelman's syndrome is an autosomal recessive salt-losing tubulopathy showing hypokalemic hypomagnesemic hypocalciuria with metabolic alkalosis and hyperreninemic hyperaldosteronism. This syndrome is caused by mutations in the SLC12A3 gene that encodes sodium-chloride cotransporter expressed at the apical membrane of renal distal convoluted tubule. Symptoms and renal outcomes of Gitelman's syndrome are, in general, mild and benign, and renal insufficiency from Gitelman's syndrome associated with long-standing hypokalemia and volume depletion is extremely rare. Herein, we report a 27-year-old male patient with Gitelman's syndrome who manifested renal failure, hypokalemia, severe metabolic alkalosis and altered mentality. About one year ago, the patient had been transferred to Seoul National University Hospital, because of unsolved hypokalemia, and was diagnosed as Gitelman's syndrome by clinical features and genetic analysis of the SLC12A3 gene. The patient carries a missense mutation at one allele of SLC12A3 gene (c.781C>T, p.Arg261Cys). His mother is also heterozygous for the same mutation and she had a history of hypokalemia. On this admission, the patient had recurrent bouts of vomiting induced by psychiatric eating disorder and showed severe volume depletion with hypotension, azotemia and metabolic alkalosis. Intense hydration therapy and emergency hemodialysis transiently improved his fluid-electrolyte imbalance and renal function. However, renal dysfunction progressively deteriorated despite the medical treatment. Our findings suggest that even in Gitelman's syndrome, constant monitoring for volume status and other comorbid conditions should be employed to prevent progressive renal injury.