The voltage-gated cardiac sodium channel, SCN5A, is responsible for the initial upstroke of the action potential. Mutations in the human SCN5A gene cause susceptibility to cardiac arrhythmias and sudden death in the long QT syndrome (LQT). In this report we characterize the genomic structure of SCN5A. SCN5A consists of 28 exons spanning approximately 80 kb on chromosome 3p21. We describe the sequences of all intron/exon boundaries and a dinucleotide repeat polymorphism in intron 16. Oligonucleotide primers based on exon-flanking sequences amplify all SCN5A exons by PCR. This work establishes the complete genomic organization of SCN5A and will enable high-resolution analyses of this locus for mutations associated with LQT and other phenotypes for which SCN5A may be a candidate gene.