A 56-year-old mentally retarded Japanese woman (intelligence quotient: 49) was admitted to our hospital with the chief complaints of headache, dizziness, vomiting, and lower limb paralysis. Laboratory tests showed severe hypokalemia, metabolic alkalosis, hypomagnesemia, and hypocalciuria. These findings suggested a diagnosis of Gitelman's syndrome (GS). We examined the thiazide-sensitive Na-Cl cotransporter (TSC) gene for the mutations that can be responsible for Gitelman's syndrome, and confirmed the diagnosis. After potassium and magnesium supplementation, her paralysis improved dramatically. The marriage of her parents was consanguineous. She had nine siblings (all with mental retardation), among whom five had died of unknown causes during childhood. Familial mental retardation has never been detected before in Gitelman's syndrome. Here we report a rare case of Gitelman's syndrome with familial mental retardation.