Two small case series of platelet mitochondrial complex I activity assays in Huntington's Disease (HD) report discrepant results. We measured platelet complex I and complex I/III activity in 21 subjects with early gene-positive HD and 14 age-matched controls. The 21 participants with HD that we studied are greater than the total of 16 in the two previously published of platelet ETS activity in HD. Reductions > 10% were excluded with 80% confidence. A systemic defect in complex I activity is not present in early HD when striatal neuronal degeneration is already present.