Statins have been suggested to protect against Alzheimer's disease (AD). Recently, however, we reported that aged dogs that underwent chronic statin treatment exhibited cognitive deficits compared with age matched controls. In human studies, blood levels of Coenzyme Q10 (CoQ10) decrease with statin use. CoQ10 is important for proper mitochondrial function and is a powerful antioxidant, two important factors for cognitive health in aging. Thus, the current study tested the hypothesis that CoQ10 levels in the serum and/or parietal cortex are decreased in statin treated dogs and are associated with poorer cognition. Six aged beagles (>8 years) were administered 80 mg/day of atorvastatin for 14.5 months and compared with placebo-treated animals. As predicted, serum CoQ10 was significantly lower in statin-treated dogs. Parietal cortex CoQ10 was not different between the two groups. However, poorer cognition was correlated with lower parietal cortex CoQ10. This study in dogs suggests that serum CoQ10 is reduced with atorvastatin treatment. CoQ10 levels in brain may be linked to impaired cognition in response to atorvastatin, in agreement with previous reports that statins may have a negative impact on cognition in the elderly.