Serum coenzyme Q10 (Q10) concentrations were evaluated in healthy male volunteers supplemented with 30 mg or 100 mg Q10 or placebo as a single daily dose for two months in a randomised, double-blind, placebo-controlled study. Median baseline serum Q10 concentration in 99 men was 1.26 mg/l (10%, 90% fractiles: 0.82, 1.83). Baseline serum Q10 concentration did not depend on age, while borderline significant positive associations were found for body weight and smoking 1-10 cigarettes/d. Supplementation with 30 mg or 100 mg Q10 resulted in median increases in serum Q10 concentration of 0.55 mg/l and 1.36 mg/l, respectively, compared with a median decrease of 0.23 mg/l with placebo. The changes in the Q10 groups were significantly different from that in the placebo group, and the increase in the 100 mg Q10 group was significantly greater than that in the 30 mg Q10 group. The change in serum Q10 concentration in the Q10 groups did not depend on baseline serum Q10 concentration, age, or body weight.