Generalized epilepsy with febrile seizures plus (GEFS+), a clinical subset of febrile seizures (FS), is characterized by frequent episodes beyond 6 years of age (FS+) and various types of subsequent epilepsy. Mutations in β1 and α_I-subunit genes of voltage-gated Na⁺ channels have been associated with GEFS+1 and 2, respectively. Here, we report a mutation resulting in an amino acid exchange (R187W) in the gene encoding the α-subunit of neuronal voltage-gated Na⁺ channel type II (Na_v1.2) in a patient with FS associated with afebrile seizures. The mutation R187W occurring on Arg¹⁸⁷, a highly conserved residue among voltage-gated Na⁺ channels, was not found in 224 alleles of unaffected individuals. Whole-cell patch clamp recordings on human embryonic kidney (HEK) cells expressing a rat wild-type (rNa_v1.2) and the corresponding mutant channels showed that the mutant channel inactivated more slowly than wild-type whereas the Na⁺ channel conductance was not affected. Prolonged residence in the open state of the R187W mutant channel may augment Na⁺ influx and thereby underlie the neuronal hyperexcitability that induces seizure activity. Even though a small pedigree could not show clear cosegregation with the disease phenotype, these findings strongly suggest the involvement of Na_v1.2 in a human disease and propose the R187W mutation as the genetic defect responsible for febrile seizures associated with afebrile seizures.