A significant proportion of patients with inborn errors of the mitochondrial respiratory chain exhibit movement disorders, particularly dystonia. Point mutations of mitochondrial DNA (mtDNA) are usually expressed systemically, and defects of platelet respiratory chain function have been described in patients with mtDNA mutations and Leber's hereditary optic neuropathy (LHON). Recent reports have documented families with dystonia in association with LHON and mtDNA complex I gene mutations. We have examined mitochondrial function in platelet mitochondria from patients with familial generalized dystonia (linked or not linked to 9q34) and sporadic focal dystonia. We confirm a previous report of a specific complex I defect in patients with sporadic focal dystonia but could not find any abnormality in patients with familial generalized dystonia, linked or not to 9q34. These results support the existence of a mitochondrial deficiency in sporadic focal dystonia and provide a biochemical dimension to the clinical and genetic distinction between focal and generalized familial dystonia.