Half of the subjects in this study will be selected by chance to receive creatine treatment for 6 months and the other half to receive placebo. Neither the subject nor the investigator will know which drug the subject is receiving, although this information will be available in case of emergency. It is anticipated that all subjects will have the choice to receive creatine after the 6 months study in an open-label study for an additional 12 months. A total of 114 patients will participate at 15 centers. Approximately 8 subjects will be enrolled at the Washington University.