An 80-year-old man was referred to our department for evaluation of repetitive loss of consciousness and faintness with hypokalemia. He had relatively low blood pressure, hypomagnesemia, hypocalciuria and chondrocalcinosis in the knee, clinically suggesting Gitelman's syndrome. A renal clearance study could not be carried out due to the patient's age and complications of the heart. Sequence analysis of the gene of thiazide-sensitive Na-Cl cotransporter (TSC) showed a heterozygous missense

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... on from C to T at 1712 base pairs from the translation start site, with resultant changes in codon 569 from alanine to valine (A569V). Treatment with oral administration of potassium chloride improved all the symptoms. Although Gitelman's syndrome has been considered to be autosomal recessive, cases of only heterozygous mutation detected have recently been reported. Therefore, the mutation found in this patient may be responsible for Gitelman's syndrome.