OBJECTIVE To screen for mutations in a Chinese pedigree affected with hypokalemic periodic paralysis. METHODS The proband and nine family members were enrolled for the analysis of CACNA1S and SCN4A gene mutations. Genomic DNA was extracted from peripheral blood samples. The coding regions of the two genes were amplified with PCR and subjected to Sanger sequencing. Potential impact of suspected mutations was predicted with Bioinformatics software. The mutations were also verified among 100 healthy controls. RESULTS The proband and 5 family members (including 5 males and 1 female) had presented with episodes of flaccid paralysis accompanied by low serum potassium. Genetic testing has identified a c.664C>T (p.Arg222Trp) mutation in the proband, which has been reported previously. The same mutation was identified in other 5 affected members from the family. No mutation of the CACNA1S gene was detected. CONCLUSION The c.664C>T mutation of the SCN4A gene probably underlies the hypokalemic periodic paralysis in this family. All patients from the family have shown a complete penetrance of the disease.