This study will help to determine the safety and efficacy of the nutritional supplement Coenzyme Q10 when added to steroids as a treatment for Duchenne muscular dystrophy (DMD). Boys with DMD who are enrolled in this study will should be on a stable dose of steroids for at least six months, and will remain on their usual dose throughout the study. They will complete two screening visits within a one-week period, and if enrolled will then have their strength tested monthly for three months before beginning therapy with Coenzyme Q10. Once Coenzyme Q10 therapy is started, participants will have their strength tested monthly for six months. Following the six month treatment period, participants will be given the option to remain on Coenzyme Q10 until the study is completed.