The mouse mutant medJ contains a splice site mutation in the neuronal sodium channel Scn8a that results in a very low level of expression. On a C57BL/6J genetic background, medJ homozygotes exhibit progressive paralysis and juvenile lethality. The C3H genetic background has an ameliorating effect, producing viable adults with a novel dystonic phenotype. The dystonic mice exhibit movement-induced, sustained abnormal postures of the trunk and limbs. A dominant modifier locus responsible for the difference between strains was mapped to a 4.5 +/- 1.3 cM interval on mouse chromosome 3. Our findings establish a role for ion channels in dystonia and demonstrate the impact of genetic background on its severity and progression. This new model suggests that SCN8A on chromosome 12q13 and SCNM1 on chromosome 1p21-1q21 may contribute to human inherited dystonia.