The effects of ubiquinone-10 (Q₁₀) supplementation on aerobic and anaerobic physical performance was examined after anaerobic training in 15 healthy men (20-34 years old, VO_2max 4.5±0.7 litre/min). 9 men were supplemented twice daily with Q₁₀ 60 mg and 6 with placebo for 20 days. Plasma creatine kinase activity (CK) was used as a marker of cell damage. No significant change was observed in CK in the placebo group. In the Q₁₀ group, plasma CK increased significantly 6 h after the anaerobic test on day 11 and remained elevated for 24 h after cessation of training. Work capacity during anaerobic tests did not differ in Q₁₀ group but increased significantly compared with day 1 in the placebo group. Results suggest that during high intensity exercise, Q₁₀ supplementation may cause increased cellular damage.