Coenzyme Q10 (CoQ10) plays an important role in bioenergetic processes and has antioxidant activity. Fifteen exercise-trained individuals (10 men and 5 women; 30-65 years) received reduced CoQ10 (Kaneka QH ubiquinol; 300 mg per day) or a placebo for four weeks in a random order, double blind, cross-over design (3 week washout). After each four-week period, a graded exercise treadmill test and a repeated cycle sprint test were performed (separated by 48 hours). Blood samples were collected before and immediately following both exercise tests and analyzed for lactate, malondialdehyde, and hydrogen peroxide. Resting blood samples were analyzed for CoQ10 (ubiquinone and ubiquinol) profile before and after each treatment period. Treatment with CoQ10 resulted in a significant increase in total blood CoQ10 (138%; P = 0.02) and reduced blood CoQ10 (168%; P = 0.02), but did not improve exercise performance (with the exception of selected individuals) or impact oxidative stress. The relationship between the percentage change in total blood CoQ10 and the cycle sprint total work (R(2) = 0.6009) was noted to be moderate to strong. We conclude that treatment with CoQ10 in healthy, exercise-trained subjects increases total and reduced blood CoQ10, but this increase does not translate into improved exercise performance or decreased oxidative stress.