Parkinson's disease (PD) is a degenerative neurological disorder. Recent studies have demonstrated reduced activity of complex I of the electron transport chain in brain and platelets from patients with PD. Platelet mitochondria from parkinsonian patients were found to have lower levels of coenzyme Q10 (CoQ10) than mitochondria from age/sex-matched controls. There was a strong correlation between the levels of CoQ10 and the activities of complexes I and II/III. Oral CoQ10 was found to protect the nigrostriatal dopaminergic system in one-year-old mice treated with MPTP, a toxin injurious to the nigrostriatal dopaminergic system. We further found that oral CoQ10 was well absorbed in parkinsonian patients and caused a trend toward increased complex I activity. These data suggest that CoQ10 may play a role in cellular dysfunction found in PD and may be a potential protective agent for parkinsonian patients.