A mutation in the DYT1 gene on chromosome 9q34 causes early-onset primary torsion dystonia with autosomal dominant inheritance but low phenotypic penetrance. The aim of the present study was to assess the functional consequences of the DYT1 gene, by comparing the electrophysiology of cortical and spinal circuits in clinically affected and unaffected carriers of the DYT1 gene mutation. We assessed intracortical inhibition (ICI), intracortical facilitation (ICF), the cortical silent period (SP) and spinal reciprocal inhibition (RI) in 10 manifesting DYT1 gene carriers (MDYT1), seven non-manifesting DYT1 gene carriers (NMDYT1) and 13 healthy controls. The MDYT1 subjects had abnormalities similar to those seen in previous studies of non-genetically characterized individuals with primary dystonia. They had reduced ICI, shorter SP and absent presynaptic phase of RI compared with the healthy controls. NMDYT1 subjects also had a significant reduction in cortical inhibition (ICI and SP), but their spinal RI was not different from controls. We conclude that clinical expression of dystonia depends on widespread electrophysiological deficits, and the presence of the DYT1 gene mutation itself leads only to a subset of these changes. This is consistent with the hypothesis that additional environmental/genetic insults may be needed to reveal clinical symptoms in DYT1 gene carriers.