Recent genetic studies have identified mutations in the genes causative for familial hypokalemic periodic paralysis (F-HypoPP) . The major locus for F-HypoPP is the skeletal muscle calcium channel alpha(1S) subunit (CACNA1S), and two predominant missense mutations were found in the same gene. A small part of other F-HypoPP families has been identified to result from the missense mutations found in the skeletal muscle sodium channel alpha subunit (SCN4A) gene. We analyzed Japanese hypokalemic periodic paralysis patients (familial, sporadic and thyrotoxic), and detected the Arg 528His mutation in two F-HypoPP families. Thus F-HypoPP is related to the functional deficiency of the skeletal muscle ion-channels.