To elucidate whether serum coenzyme Q10 levels are related with the risk for multiple sclerosis (MS) or are a marker for the activity of the disease, we compared serum levels of coenzyme Q10 and the coenzyme Q10/cholesterol ratio, in 31 patients with MS (during exacerbations) and 19 matched controls using a high performance liquid chromatography technique. The mean serum coenzyme Q10 levels and the coenzyme Q10/cholesterol ratio did not differ significantly between the 2 study groups. The values did not correlate with age, age at onset, and duration of the disease. These results suggest that serum coenzyme Q10 concentrations are unrelated with the risk for MS and are not a useful marker of activity of the disease.