We report a precocious and atypical form of hypokalaemic periodic paralysis, with clinical manifestations at birth and first episodes of paralysis occurring as early as 1 year of age, although onset of this disease usually occurs between 5-35 years. Extensive molecular analysis showed that the disease was caused by a novel de novo p.Arg897Ser mutation in the CACNA1S gene. The mutation mapped to a new region of the protein, the S4 voltage sensing segment of domain III, at odds with previously reported mutations that exclusively affected domains II and IV.