It has been suggested that dystonia is caused by an autosomal gene with reduced penetrance and a consequent biochemical abnormality affecting cell activity within the basal ganglia. No consistent biochemical disturbance has been identified. In the present study, activities of the mitochondrial electron transfer complexes were measured in platelets of 31 patients with idiopathic dystonia. Enzyme assays of these patients were compared to measurements in 28 control subjects. A significant decrease of complex I activity was observed in the majority of the patients, whereas the activities of other electron transfer complexes were normal. The severity of the complex I defect was more pronounced in patients with the segmental or generalized form than in those with focal dystonia. Complex I activity was not age dependent in the patients or control subjects. Although the electron pathway in complex I is disturbed in patients with idiopathic dystonia, complex I protein content seems to be normal. Whether abnormalities of complex I activity play a role in the pathogenesis of idiopathic dystonia remains to be determined.