The objective of this study is to determine whether creatine slows disease progression in subjects with amyotrophic lateral sclerosis (ALS). ALS is a progressive uniformly lethal neurodegenerative disorder for which there is no known cure. Recent genetic and biochemical studies implicate free radical toxicity, glutamate excitotoxicity and mitochondrial dysfunction as possible causes of familial ALS (FALS) and sporadic ALS (SALS). It has been hypothesized that in ALS there may be involvement of oxidative free radical damage and impaired mitochondrial energy metabolism that could in turn lead to excitotoxic cell death. Creatine, an agent that improves mitochondrial function, has been shown to be neuroprotective in animal models of ALS and Huntington's disease.