In three families with hypokalemic periodic paralysis (HOPP) we have confirmed the presence of a missense mutation (arginine 528 to histidine) within the gene CACNL1A3 encoding the alpha 1 subunit of the L-type, voltage-sensitive calcium channel. Additionally, we have identified two novel polymorphisms within this gene located in close proximity to the mutation. Haplotype analysis using these and other polymorphisms indicates that these families do not share a common mutation due to a founder effect. Rather, an HOPP phenotype has arisen in these families from three separate but identical mutations.